rockchip: add support for veyron-speedy (ASUS Chromebook C201)

This adds support for the ASUS C201, a RK3288-based clamshell
device. The device tree comes from linus's linux tree at
3f16503b7d2274ac8cbab11163047ac0b4c66cfe. The SDRAM parameters
are for 4GB Samsung LPDDR3, decoded from coreboot's
src/mainboard/google/veyron/sdram_inf/sdram-lpddr3-samsung-4GB.inc

Signed-off-by: Marty E. Plummer <hanetzer@startmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>