ARM: UniPhier: decrease pre-reloc malloc area size

ARM: UniPhier: decrease pre-reloc malloc area size

In the current implementation of the boot sequence of UniPhier
platform, 32KB temporary RAM is available before relocation.
The malloc area and the stack shares the 32KB area.

With CONFIG_SYS_MALLOC_F_LEN set to 0x7000 (28KB), only 0x1000 (4KB)
is left for the stack.  In some use cases, the system hangs up
with stack over-flow.

Even with driver-model UART enabled, the malloc area of 0x2000 (8KB)
should be enough.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>