mfd: axp20x: Extend axp20x to support axp288 pmic

X-Powers AXP288 is a customized PMIC for Intel Baytrail-CR platforms. Similar
to AXP202/209, AXP288 comes with USB charger, more LDO and BUCK channels, and
AD converters. It also provides extended status and interrupt reporting
capabilities than the devices currently supported in axp20x.c.

In addition to feature extension, this patch also adds ACPI binding for
enumeration.

This consolidated driver should support more X-Powers' PMICs in both device
tree and ACPI enumerated platforms.

Signed-off-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Reviewed-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>