PM: Simplify kernel/power/Kconfig

'n' defaults are pretty pointless and actually bogus when used with
prompt-less config options.

The "bool"/"default y" pair with no prompt can be expressed more
compactly using def_bool.

[rjw: Rebased on top of earlier patches modifying this file.]

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>