ARM: UniPhier: enable Driver Model and UART on SPL

Enable CONFIG_SPL_DM and CONFIG_SPL_SERIAL_SUPPORT, which provide
Driver Model UART support on SPL.

CONFIG_SYS_SPL_MALLOC_{START,SIZE} should be dropped because simple
malloc is preferred on SPL.  Dlmalloc requires some static variables
on .data section that is not available yet for NOR boot mode etc.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>