mfd: axp20x: Preserve other control bits when powering off

axp20x_power_off() triggers shutdown by writing AXP20X_OFF (BIT(7)) to
the power-off control register with regmap_write(), which rewrites the
whole register and clears other control bits in it.

On the AXP221/AXP223 (and the register-compatible AXP228) - that
register also holds the CHGLED auto-control bit.
Clearing it during an orderly shutdown disables the hardware charge
indicator, so the charge LED stays dark while the board is powered off
and charging. Other variants keep unrelated configuration in the same
register too.

Set only the power-off bit with regmap_set_bits() and leave the rest of
the register untouched. The shutdown register is readable on every
variant, so the read-modify-write should be safe.

Tested on a ClockworkPi uConsole (Raspberry Pi Compute Module 4, AXP228,
which enumerates as AXP221): register AXP20X_OFF_CTRL reads 0x08 at
runtime, so the old code left it 0x80 whereas setting only BIT(7)
leaves 0x88. Writing 0x88 at power-off enables the charge LED
while still powering off the PMIC.

Signed-off-by: Oleg Proshkin <oleg.pro171@gmail.com>
Reviewed-by: Chen-Yu Tsai <wens@kernel.org>
Link: https://patch.msgid.link/20260611154629.76607-1-oleg.pro171@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>

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