ARM: dts: berlin2cd: add Valve Steam Link board

Valve Steam Link is a consumer device built around the Marvell BG2CD SoC.
This board file enables the UART, USB and Ethernet interfaces as well as
internal I2C and SDIO, and adds SoC voltage regulator and board-specific
GPIO restart method info.

Cc: Sam Lantinga <saml@valvesoftware.com>
Signed-off-by: Alexander Monakov <amonakov@ispras.ru>
Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>