powerpc/8xx: Always pin kernel text TLB

There is no big poing in not pinning kernel text anymore, as now
we can keep pinned TLB even with things like DEBUG_PAGEALLOC.

Remove CONFIG_PIN_TLB_TEXT, making it always right.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Drop ifdef around mmu_pin_tlb() to fix build errors]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/203b89de491e1379f1677a2685211b7c32adfff0.1606231483.git.christophe.leroy@csgroup.eu