ARC: HSDK-4xD: add initial board support

Add initial HSDK-4xD board support.
The ARC HS4x/HS4xD Development Kit includes a multicore ARC HS4xD-based
chip that integrates a wide range of interfaces including Ethernet,
HDMI, WiFi, Bluetooth, USB, SDIO, I2C, SPI, UART, I2S, ADC, PWM and
GPIO, as well as a Think Silicon GPU.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/board/synopsys/hsdk/MAINTAINERS b/board/synopsys/hsdk/MAINTAINERS
index e22bd1e..d385951 100644
--- a/board/synopsys/hsdk/MAINTAINERS
+++ b/board/synopsys/hsdk/MAINTAINERS
@@ -1,5 +1,6 @@
-HSDK BOARD
+HSDK BOARDs
 M:	Eugeniy Paltsev <paltsev@synopsys.com>
 S:	Maintained
 F:	board/synopsys/hsdk/
 F:	configs/hsdk_defconfig
+F:	configs/hsdk_4xd_defconfig