ARM: shmobile: r8a73a4: add a clock alias for the DMAC in DT mode

Devices, initialised from the Device Tree and from platform code usually
have different names. This patch adds a clock alias for DMAC on r8a73a4
in DT mode.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>