poplar: increase SYS_MALLOC_F_LEN for DM_FLAG_PRE_RELOC support

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 <[email protected]> for reporting, and Bin Meng
<[email protected]> for trouble shooting.

Reported-by: Andreas Färber <[email protected]>
Signed-off-by: Shawn Guo <[email protected]>
Reviewed-by: Bin Meng <[email protected]>
Tested-by: Andreas Färber <[email protected]>