ARM: UniPhier: set CONFIG_SYS_MALLOC_F to the global default value

It is true that malloc is necessary for Driver Model before
relocation, but there is no good reason to reserve the malloc
space more than enough.  The default value 0x400 works well.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Reviewed-by: Simon Glass <sjg@chromium.org>