mfd: axp20x: Add a cell for the usb power_supply part of the axp20x PMICs

Add a cell for the usb power_supply part of the axp20x PMICs.

Note that this cell is only for the usb power_supply part and not the
ac-power / battery-charger / rtc-backup-bat-charger bits.

Depending on the board each of those must be enabled / disabled separately
in devicetree as most boards do not use all 4. So in dt each one needs its
own child-node of the axp20x node. Another reason for using separate child
nodes for each is so that other devicetree nodes can have a power-supply
property with a phandle referencing a node representing a single
power-supply.

The decision to use a separate devicetree node for each is reflected on
the kernel side by each getting its own mfd-cell / platform_device and
platform-driver.

Note this commit also makes some whitespace changes to the intialization
of existing cells in axp20x_cells, these are pure whitespace changes,
functionally nothing changes.

Signed-off-by: Bruno Prémont <bonbons@linux-vserver.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>