MIPS: Support for targetting MIPSr6

Add support for targetting MIPS32r6 & MIPS64r6 systems, in the same way
that we currently select release 1 or release 2 targets. MIPSr6 is not
entirely backwards compatible with earlier releases of the architecture.
Some instructions are encoded differently, some are removed, some are
reused, so it is not practical to run U-Boot built for earlier revisions
on a MIPSr6 system. Update their Kconfig help text to reflect that.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
diff --git a/arch/mips/Makefile b/arch/mips/Makefile
index 94f7e16..655a493 100644
--- a/arch/mips/Makefile
+++ b/arch/mips/Makefile
@@ -19,8 +19,10 @@
 # Optimize for MIPS architectures
 arch-$(CONFIG_CPU_MIPS32_R1) += -march=mips32 -Wa,-mips32
 arch-$(CONFIG_CPU_MIPS32_R2) += -march=mips32r2 -Wa,-mips32r2
+arch-$(CONFIG_CPU_MIPS32_R6) += -march=mips32r6 -Wa,-mips32r6
 arch-$(CONFIG_CPU_MIPS64_R1) += -march=mips64 -Wa,-mips64
 arch-$(CONFIG_CPU_MIPS64_R2) += -march=mips64r2 -Wa,-mips64r2
+arch-$(CONFIG_CPU_MIPS64_R6) += -march=mips64r6 -Wa,-mips64r6
 
 # Allow extra optimization for specific CPUs/SoCs
 tune-$(CONFIG_MIPS_TUNE_4KC) += -mtune=4kc