ARM: shmobile: genmai-reference: Enable MTU2 in device tree

No more device needs to be added from platform code when booting the
reference platform, move MTU2 registration from setup-r7s72100.c to
board-genmai.c and remove the now empty r7s72100_add_dt_devices()
function.

As the genmai_add_standard_devices() function is now identical to the
default init_machine implementation, remove it as well.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>