sunxi: orangepi_pc: Add support for eMMC found on the Orange Pi PC Plus

The Plus variant of the Orange Pi PC has an eMMC, add support for this.

Note we are using the same u-boot defconfig / dts for both the regular
Orange Pi PC as well as the Orange Pi PC Plus.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>