dt-bindings: mfd: axp20x: Add "xpowers,master-mode" property for AXP806 PMICs

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. To let the device tree describe which mode of operation is needed,
this patch introduces a new property "xpowers,master-mode".

Fixes: 204ae296 ("mfd: axp20x: Add bindings for AXP806 PMIC")
Signed-off-by: Rask Ingemann Lambertsen <rask@formelder.dk>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>