ARM: shmobile: r8a7791: Correct SYS-DMAC clock defines

ARM: shmobile: r8a7791: Correct SYS-DMAC clock defines

R-Car M2 has two MSTP bits for SYS-DMAC, not one.
Also bring the naming in sync with the documentation.

This issue was introduced in v3.14, in commit
4d8864c9e94ec727f1c675b9f6921525c360334b ("ARM: shmobile: r8a7791: Add
clock index macros for DT sources").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
(cherry picked from commit a505daa501dc3f02b08d0b48b049bce719299b74)
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>