boards:sunxi: Add MSI Primo73 tablet

This is a 7" tablet with Allwinner A20 SoC. More details at
    http://linux-sunxi.org/MSI_Primo73

DRAM settings are the same as in the pre-installed Android firmware.
Reliability has been verified by using the lima-memtester tool in two
configurations:
  1. the default "dram_clock=384MHz, dcdc3=1.250V" settings
  2. artificially adjusted "dram_clock=408MHz, dcdc3=1.225V" settings
     (DRAM clock frequency increased by one step and dcdc3 voltage
     reduced by one step).
No problems have been detected in either of them after several hours
of running tests.

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>