Porter is an entry level development board based on R-Car M2 SoC (R8A7791)

This commit supports the following peripherals:
- SCIF, I2C, Ethernet, QSPI, SD, USB Host
Signed-off-by: NVladimir Barinov <vladimir.barinov@cogentembedded.com>
Signed-off-by: NNobuhiro Iwamatsu <iwamatsu@nigauri.org>