power: pm2301-charger: use __maybe_unused to hide pm functions

The pm2301 charger driver uses nested #ifdefs to check for both
CONFIG_PM and CONFIG_PM_SLEEP in an attempt to hide its
suspend and runtime-pm operations when they are unused, but
it does not hide the clear_lpn_pin() function in the same
way, so we get a build warning when everything is
disabled:

drivers/power/pm2301_charger.c:123:13: error: 'clear_lpn_pin' defined but not used [-Werror=unused-function]

This removes all the #ifdef and instead uses __maybe_unused
annotations to let the compiler know it can silently drop
the function definition.

For the PM2XXX_PM_OPS, we can use an IS_ENABLED() check
to avoid defining the structure when CONFIG_PM is not set without
the #ifdef.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Reichel <sre@kernel.org>