MIPS: add possibility to setup initial stack and global data in SRAM

MIPS: add possibility to setup initial stack and global data in SRAM

This adds a new Kconfig option CONFIG_MIPS_INIT_STACK_IN_SRAM which
a SoC can select if it supports some kind of SRAM. Together with
CONFIG_SYS_INIT_SP_ADDR the initial stack and global data can be
set up in that SRAM. This can be used to provide a C environment
also for lowlevel_init().

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>