mips: add an option to support initialize SRAM for initial stack

mips: add an option to support initialize SRAM for initial stack

Currently CONFIG_MIPS_INIT_STACK_IN_SRAM assumes the memory space for the
initial stack can be used directly. However on some platform the SRAM needs
initialization, e.g. lock cache.

This patch adds an option to allow a new function mips_sram_init() being
called before setup_stack_gd.

Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>