MSI_Primo81_defconfig: enable USB OTG port and keyboard support

The MSI Primo 81 is an Allwinner A31s-based tablet on which the
OTG port is the only accessible USB interface.  The existing
defconfig had VGA console on the LCD enabled, but was missing
keyboard support because the prerequisites for that (sunxi MUSB
support and AXP221 GPIO support) had not been available before.
All previously missing dependencies are available now, so this
patch enables keyboard support and its prerequisites in the
defconfig.

Signed-off-by: Karsten Merker <merker@debian.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>