corstone700: updating the kernel arguments to support initramfs

In the context of enabling initramfs this change makes
the kernel arguments compatible with the initramfs requirements

Change-Id: Ifa955a5790ae1398fd8ad9ca1c8272f019c121a6
Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>

fdts/corstone700.dts

@@ -15,8 +15,6 @@
 
 	chosen {
 		bootargs = "console=ttyAMA0 \
-		root=mtd:physmap-flash.0 \
-		ro \
 		loglevel=9";
 	};