powerpc: Add generic PAGE_SIZE config symbols

Other arches (sh, mips, hexagon) use standard names for PAGE_SIZE
related config symbols.

Add matching symbols for powerpc, which are enabled by default but
depend on our architecture specific PAGE_SIZE symbols.

This allows generic/driver code to express dependencies on the PAGE_SIZE
without needing to refer to architecture specific config symbols.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220505125123.2088143-1-mpe@ellerman.id.au