allwinner: power: fix DRIVEVBUS pin setup

The DRIVEVBUS pin setup was broken in two ways:
- To configure this pin as an output pin, one has to *clear* the bit in
  register 0x8f. It is 0 by default, but rebooting from Linux might have
  left this bit set.
- Doing this just configures the pin as an output pin, but doesn't
  actually drive power to it. This is done via bit 2 in register 0x30.

Fix the routine to both properly configure the pin and drive power to
it. Add an axp_clrsetbits() helper on the way.

Now this isn't really perfect, still:
We only need to setup the PMIC power rails that are needed for U-Boot.
DRIVEVBUS typically controls the VBUS voltage for the host function of
an USB-OTG port, something we typically don't want in U-Boot (fastboot,
using the USB *device* functionality, is much more common). The
BananaPi-M64 uses the regulator in this way, but the Remix Mini PC
actually controls the power of both its USB ports via this line.

Technically we should differentiate here: if DRIVEVBUS controls a
microUSB-B socket, the power should stay off, any host-type A sockets
should be supplied, though.
For now just always enable the power, that shouldn't really hurt the
USB-OTG functionality anyway.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>

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