mfd: core: Remove .enable() and .disable() callbacks

With commit dd77f5fa97d3 ("mfd: Remove toshiba tmio drivers") the last
mfd driver that implements these callbacks is gone and since commit
652719b1 ("w1: remove ds1wm driver") the last user is gone. The
corresponding functions mfd_cell_enable() and mfd_cell_disable() are
also unused (since commit 0ca222c8 ("leds: Remove asic3 driver")).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230308091257.2404932-1-u.kleine-koenig@pengutronix.de