MIPS: add support for pre-relocation malloc

Implement MIPS specific setup of the gd_t structure to support
pre-relocation malloc. If CONFIG_SYS_MALLOC_F_LEN is specified,
a memory area will be reserved after the initial stack area and
the gd->malloc_base pointer will be initialized.

After this patch the new driver model can be used on MIPS.

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