ARM: shmobile: Mount root file systems in r/w mode for all DT platforms

The shmobile DT files available in the kernel are reference
implementations intended to be used as sample code, as well as for
development. As such, it makes sense to mount the root file system in
read/write mode by default.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>