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>