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 3a7c45f6 ("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>