MIPS: RM7000: Add support for tertiary cache

Add support for the external T-cache interface. Allow for platform
independent size probing from 512KB to 8MB in powers of two.

Signed-off-by: Ricardo Mendoza <ricmm@gentoo.org>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1477/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>