powerpc/32s: Rename PTE_SIZE to PTE_T_SIZE

PTE_SIZE means PTE page table size in most placed, whereas
in hash_low.S in means size of one entry in the table.

Rename it PTE_T_SIZE, and define it directly in hash_low.S
instead of going through asm-offsets.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/83a008a9fd6cc3f2bbcb470f592555d260ed7a3d.1623063174.git.christophe.leroy@csgroup.eu