ARM: shmobile: r8a7791: Add clocks

Declare all core clocks and DIV6 clocks, as well as all MSTP clocks
currently used by r8a7791 boards.

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