mips: add a option to support not reserving malloc space on initial stack

mips: add a option to support not reserving malloc space on initial stack

The initial stack on some platforms is too small to hold a large malloc
space. This patch adds a option to allow these platforms not reserving the
malloc space on initial stack. These platforms should set the malloc base
after DRAM is usable.

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