MIPS: Allow FTLB to be turned on for CPU_P5600

Allow FTLB to be turned on or off for CPU_P5600 as well as CPU_PROAPTIV.
The existing if statement is converted into a switch to allow for future
expansion.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6411/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>