powerpc: indent to improve Kconfig readability

Indent a Kconfig continuation line to improve readability.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ff8729c1-3a4b-c720-48ba-a1a42b0ef892@infradead.org