powerpc/mm: Simplify page size kconfig dependencies

For config options with only a single value, guarding the single value
with 'if' is the same as adding a 'depends' statement. And it's more
standard to just use 'depends'.

And if the option has both an 'if' guard and a 'depends' we can collapse
them into a single 'depends' by combining them with &&.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>