mfd: axp20x: Add CHRG_CTRL1/2/3 to writeable regs for AXP20X/AXP22X

The CHRG_CTRL1 and CHRG_CTRL2 registers are made for controlling
different battery charging settings such as the constant current charge
value.

The AXP22X also have a third register CHRG_CTRL3 which has settings for
battery charging too.

This adds the CHRG_CTRL1, CHRG_CTRL2 and CHRG_CTRL3 registers to the
list of writeable registers for AXP20X and AXP22X PMICs.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>