powerpc32/booke: consistently return phys_addr_t in __pa()

When CONFIG_RELOCATABLE=y is set, VIRT_PHYS_OFFSET is a 64bit variable,
thus __pa() returns as 64bit value.
But when CONFIG_RELOCATABLE=n, __pa() returns 32bit value.

When CONFIG_PHYS_64BIT is set, __pa() should consistently return as
64bit value irrelevant to CONFIG_RELOCATABLE.
So we'd make __pa() consistently return phys_addr_t, which is 64bit
when CONFIG_PHYS_64BIT is set.

Signed-off-by: Bai Yingjie <byj.tea@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200106042957.26494-1-yingjie_bai@126.com