powerpc: Make PPC_IRQ_SOFT_MASK_DEBUG depend on PPC64

32-bit platforms don't have irq soft masking.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210623032909.826010-1-npiggin@gmail.com