powerpc/irq: Add stub irq_soft_mask_return() for PPC32

To allow building syscall_64.c smoothly on PPC32, add stub version
of irq_soft_mask_return().

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/9b9f62c5e2e63cc121fd749a923aaaee92ee0da4.1612796617.git.christophe.leroy@csgroup.eu