mfd: axp20x: Do not sleep in the power off handler

Since commit 856c288b ("ARM: Use do_kernel_power_off()"), the
function axp20x_power_off() now runs inside a RCU read-side critical
section, so it is not allowed to call msleep(). Use mdelay() instead.

Fixes: 856c288b ("ARM: Use do_kernel_power_off()")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221105212909.6526-1-samuel@sholland.org