mfd: axp20x: Add support for dts property "xpowers,master-mode"

commit b101829a029a ("mfd: axp20x: Fix AXP806 access errors on cold boot")
was intended to fix the case where a board uses an AXP806 in slave mode,
but the boot loader leaves it in master mode for lack of AXP806 support.
But now the driver breaks on boards where the PMIC is operating in master
mode. This patch lets the driver use the new device tree property
"xpowers,master-mode" to set the correct operating mode for the board.

Fixes: 8824ee85 ("mfd: axp20x: Add support for AXP806 PMIC")
Signed-off-by: Rask Ingemann Lambertsen <rask@formelder.dk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>