Input: spear-keyboard - use __maybe_unused to hide pm functions

The spear keyboard driver uses #ifdef CONFIG_PM to hide its
power management functions, but then uses references from
SIMPLE_DEV_PM_OPS that are only present if both CONFIG_PM
and CONFIG_PM_SLEEP are set, resulting in a warning about unused
functions:

drivers/input/keyboard/spear-keyboard.c:292:12: error: 'spear_kbd_suspend' defined but not used [-Werror=unused-function]
drivers/input/keyboard/spear-keyboard.c:345:12: error: 'spear_kbd_resume' defined but not used [-Werror=unused-function]

This removes the #ifdef and instead uses a __maybe_unused
annotation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>