mips: Allow overriding start.S in SPL

Certain chips, like the JZ47xx, have extreme size constraints on the
SPL size and require custom start.S . Allow overriding the start.S
the same way ARM MXS does it.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Paul Burton <paul.burton@imgtec.com>