fex:a20: Add fex file for MSI Primo73 tablet

More details at http://linux-sunxi.org/MSI_Primo73

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
fex:a31s: Add fex file for MSI Primo81 tablet

More details at http://linux-sunxi.org/MSI_Primo81

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