poplar: increase SYS_MALLOC_F_LEN for DM_FLAG_PRE_RELOC support

There is a regression seen on Poplar platform, which doesn't even show
a U-Boot version banner on booting.  It turns out that due to landing
of commit 3a7c45f6a772 ("simple-bus: add DM_FLAG_PRE_RELOC flag to
simple-bus driver"), we need to increase SYS_MALLOC_F_LEN from its
default size 0x400, as pre-relocation requires more memory there.  Let's
increase SYS_MALLOC_F_LEN to 0x4000 to fix the regression.

Thanks to Andreas Färber <afaerber@suse.de> for reporting, and Bin Meng
<bmeng.cn@gmail.com> for trouble shooting.

Reported-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Andreas Färber <afaerber@suse.de>
diff --git a/configs/poplar_defconfig b/configs/poplar_defconfig
index dfde654..62eaaee 100644
--- a/configs/poplar_defconfig
+++ b/configs/poplar_defconfig
@@ -1,6 +1,7 @@
 CONFIG_ARM=y
 CONFIG_TARGET_POPLAR=y
 CONFIG_SYS_TEXT_BASE=0x37000000
+CONFIG_SYS_MALLOC_F_LEN=0x4000
 CONFIG_NR_DRAM_BANKS=2
 CONFIG_IDENT_STRING="poplar"
 CONFIG_DISTRO_DEFAULTS=y