Finish conversion of CONFIG_SYS_CLK_FREQ to Kconfig

In order to finish moving this symbol to Kconfig for all platforms, we
need to do a few more things.  First, for all platforms that define this
to a function, introduce CONFIG_DYNAMIC_SYS_CLK_FREQ, similar to
CONFIG_DYNAMIC_DDR_CLK_FREQ and populate clock_legacy.h.  This entails
also switching all users from CONFIG_SYS_CLK_FREQ to get_board_sys_clk()
and updating a few preprocessor tests.

With that done, all platforms that define a value here can be converted
to Kconfig, and a fall-back of zero is sufficiently safe to use (and
what is used today in cases where code may or may not have this
available).  Make sure that code which calls this function includes
<clock_legacy.h> to get the prototype.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/ls2088ardb_qspi_SECURE_BOOT_defconfig b/configs/ls2088ardb_qspi_SECURE_BOOT_defconfig
index a299db9..c49d163 100644
--- a/configs/ls2088ardb_qspi_SECURE_BOOT_defconfig
+++ b/configs/ls2088ardb_qspi_SECURE_BOOT_defconfig
@@ -20,6 +20,7 @@
 CONFIG_FIT_VERBOSE=y
 CONFIG_OF_BOARD_SETUP=y
 CONFIG_OF_STDOUT_VIA_ALIAS=y
+CONFIG_DYNAMIC_SYS_CLK_FREQ=y
 CONFIG_QSPI_BOOT=y
 CONFIG_BOOTDELAY=10
 CONFIG_BOOTCOMMAND="sf probe 0:0; sf read 0x806c0000 0x6c0000 0x40000; env exists mcinitcmd && env exists secureboot && esbc_validate 0x806C0000; sf read 0x80d00000 0xd00000 0x100000; env exists mcinitcmd && fsl_mc lazyapply dpl 0x80d00000; run distro_bootcmd;run qspi_bootcmd; env exists secureboot && esbc_halt;"