rockchip: Add basic support for phyCORE-RK3288 SoM based carrier board

The phyCORE-RK3288 is a SoM (System on Module) containing a RK3288 SoC.
The module can be connected to different carrier boards.
It can be also equipped with different RAM, SPI flash and eMMC variants.
The Rapid Development Kit option is using the following setup:

  - 1 GB DDR3 RAM (2 Banks)
  - 1x 4 KB EEPROM
  - DP83867 Gigabit Ethernet PHY
  - 16 MB SPI Flash
  - 4 GB eMMC Flash

Add basic support for the PCM-947 carrier board, a RK3288 based development
board made by PHYTEC. This board works in a combination with
the phyCORE-RK3288 System on Module.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>