ARM: mach-shmobile: Use DT_MACHINE for mackerel

Use DT_MACHINE_START() on the sh7372 based mackerel board.

Also include a tiny DTS file to describe the board and update the
Kconfig dependencies to select CONFIG_USE_OF.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Simon Horman <horms@verge.net.au>