MIPS: Add platform callback before initializing the L2 cache

Allow platforms to perform platform-specific steps before configuring
the L2 cache. This is necessary for platforms with CM3 since the L2
parameters no longer live in the Config2 register.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10642/


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