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>

arch/mips/Makefile

@@ -4,6 +4,12 @@
 
 head-y := arch/mips/cpu/start.o
 
+ifeq ($(CONFIG_SPL_BUILD),y)
+ifneq ($(CONFIG_SPL_START_S_PATH),)
+head-y := $(CONFIG_SPL_START_S_PATH:"%"=%)/start.o
+endif
+endif
+
 libs-y += arch/mips/cpu/
 libs-y += arch/mips/lib/