kernel: 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>

target/linux/generic/patches-4.4/308-mips32r2_tune.patch

@@ -0,0 +1,17 @@
+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) \