mfd: axp20x: Fix AXP806 access errors on cold boot

The AXP806 supports either master/standalone or slave mode.
Slave mode allows sharing the serial bus, even with multiple
AXP806 which all have the same hardware address.

This is done with extra "serial interface address extension",
or AXP806_BUS_ADDR_EXT, and "register address extension", or
AXP806_REG_ADDR_EXT, registers. The former is read-only, with
1 bit customizable at the factory, and 1 bit depending on the
state of an external pin. The latter is writable. Only when
the these device addressing bits (in the upper 4 bits of the
registers) match, will the device respond to operations on
its other registers.

The AXP806_REG_ADDR_EXT was previously configured by Allwinner's
bootloader. Work on U-boot SPL support now allows us to switch
to mainline U-boot, which doesn't do this for us. There might
be other bare minimum bootloaders out there which don't to this
either. It's best to handle this in the kernel.

This patch sets AXP806_REG_ADDR_EXT to 0x10, which is what we
know to be the proper value for a standard AXP806 in slave mode.
Afterwards it will reinitialize the regmap cache, to purge any
invalid stale values.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>