Add -mtune=34kc to MIPS CFLAGS when building for mips32r2
This provides a good tradeoff across at least 24Kc-74Kc, while also
producing smaller code.

Signed-off-by: Felix Fietkau <nbd@nbd.name>

--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -145,7 +145,7 @@ cflags-$(CONFIG_CPU_R4X00)	+= -march=r46
 cflags-$(CONFIG_CPU_TX49XX)	+= -march=r4600 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS32_R1)	+= $(call cc-option,-march=mips32,-mips32 -U_MIPS_ISA -D_MIPS_ISA=_MIPS_ISA_MIPS32) \
 			-Wa,-mips32 -Wa,--trap
-cflags-$(CONFIG_CPU_MIPS32_R2)	+= $(call cc-option,-march=mips32r2,-mips32r2 -U_MIPS_ISA -D_MIPS_ISA=_MIPS_ISA_MIPS32) \
+cflags-$(CONFIG_CPU_MIPS32_R2)	+= $(call cc-option,-march=mips32r2 -mtune=34kc,-mips32r2 -U_MIPS_ISA -D_MIPS_ISA=_MIPS_ISA_MIPS32) \
 			-Wa,-mips32r2 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS32_R6)	+= -march=mips32r6 -Wa,--trap
 cflags-$(CONFIG_CPU_MIPS64_R1)	+= $(call cc-option,-march=mips64,-mips64 -U_MIPS_ISA -D_MIPS_ISA=_MIPS_ISA_MIPS64) \