ARM: shmobile: r8a7790: Add DMAC clocks to DT

Add the SYS-DMAC0 and SYS-DMAC1 clocks to the MSTP2 clock node. They
will be used by the upcoming DMAC DT nodes.

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