ARM: shmobile: r8a7791: Remove legacy code

All r8a7791 boards are now used with multiplatform kernels only. We can
remove all the unused r8a7791 legacy device and clock registration code.

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