mfd: axp20x: Add missing registers, and mark more registers volatile

Add an extra set of registers which is necessary tu support the PMICs
battery charger function, and mark registers which contain status bits,
gpio status, and adc readings as volatile.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>