powerpc/powernv: Kconfig: Replace single quotes

Replace single quotes with double quotes which seems to be the convention
for strings.

Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220520115229.147368-1-juergh@canonical.com