rockchip: add basic support for evb-rk3288 board

evb-3288 board RK3288-based development board with 2 USB ports, HDMI,
VGA, micro-SD card, audio, WiFi and Gigabit Ethernet. It also includes
on-board 8G eMMC and 2GB of SDRAM. Expansion connector provide access to
display pins, I2C, SPI, UART and GPIOs. This add some basic files
required to allow the board to output serial messaged and can run
command(mmc info etc).

evb-rk3288 also supports booting from eMMC or SD card, the default is eMMC.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>