ARC: Add support for EM Development Kit board

Synopsys DesignWare ARC EM Development Kit (ARC EMDK) is
an FPGA-based development platform from Synopsys aimed to speed-up
development of software for ARC EM cores and entire subsystems based on
ARC EM like Data Fusion, Secure and Sensor & Control subsystems.

U-Boot is supposed to be used as a primary bootloader on EMDK allowing
users to easily load and start their application from micro-SD card.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>