arm: rmobile: Add SILK board support

SILK is an entry level development board based on R-Car E2 SoC (R8A7794)

This commit supports the following peripherals:
- SCIF, I2C, Ethernet, QSPI, MMC, USB Host

Signed-off-by: Vladimir Barinov <vladimir.barinov@cogentembedded.com>
Reviewed-by: Tom Rini <trini@ti.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
diff --git a/configs/silk_defconfig b/configs/silk_defconfig
new file mode 100644
index 0000000..515ee33
--- /dev/null
+++ b/configs/silk_defconfig
@@ -0,0 +1,3 @@
+CONFIG_ARM=y
+CONFIG_RMOBILE=y
+CONFIG_TARGET_SILK=y