MIPS: Use CPHYSADDR to implement mips32 virt_to_phys

MIPS: Use CPHYSADDR to implement mips32 virt_to_phys

Use CPHYSADDR to implement the virt_to_phys function for converting from
a virtual to a physical address for MIPS32, much as is already done for
MIPS64. This allows for virt_to_phys to work regardless of whether the
address being translated is in kseg0 or kseg1, unlike the previous
subtraction based approach which only worked for addresses in kseg0.
This allows for drivers to provide an address to virt_to_phys without
needing to manually ensure that kseg1 addresses are converted to
equivalent kseg0 addresses first.

This patch is equivalent to this Linux patch currently waiting to be
reviewed & merged:

    https://patchwork.linux-mips.org/patch/12564/

Signed-off-by: Paul Burton <paul.burton@imgtec.com>