arm: Add support for semihosting for armv8 fastmodel targets.

The armv8 ARM Trusted Firmware (ATF) can be used to load various ATF
images and u-boot, and does this for virtual platforms by using
semihosting. This commit extends this idea by allowing u-boot to also
use semihosting to load the kernel/ramdisk/dtb. This eliminates the need
for a bootwrapper and produces a more realistic boot sequence with
virtual models.

Though the semihosting code is quite generic, support for armv7 in
fastmodel is less useful due to the wide range of available silicon
and the lack of a free armv7 fastmodel, so this change contains an
untested armv7 placeholder for the service trap opcode.

Please refer to doc/README.semihosting for a more detailed description
of semihosting and how it is used with the armv8 virtual platforms.

Signed-off-by: Darwin Rambo <drambo@broadcom.com>
Cc: trini@ti.com
Cc: fenghua@phytium.com.cn
Cc: bhupesh.sharma@freescale.com
diff --git a/boards.cfg b/boards.cfg
index 8e2db82..33435de 100644
--- a/boards.cfg
+++ b/boards.cfg
@@ -44,6 +44,7 @@
 ###########################################################################################################
 
 Active  aarch64     armv8          -           armltd          vexpress64          vexpress_aemv8a                       vexpress_aemv8a:ARM64                                                                                                             David Feng <fenghua@phytium.com.cn>
+Active  aarch64     armv8          -           armltd          vexpress64          vexpress_aemv8a_semi                  vexpress_aemv8a:ARM64,SEMIHOSTING,BASE_FVP                                                                                        Steve Rae <srae@broadcom.com>
 Active  arc         arc700         -           synopsys        -                   axs101                                -                                                                                                                                 Alexey Brodkin <abrodkin@synopsys.com>
 Active  arc         arc700         -           synopsys        <none>              arcangel4                             -                                                                                                                                 Alexey Brodkin <abrodkin@synopsys.com>
 Active  arc         arc700         -           synopsys        <none>              arcangel4-be                          -                                                                                                                                 Alexey Brodkin <abrodkin@synopsys.com>