ARM: shmobile: r8a7791 Arch timer device tree node

Add r8a7791 arch timer device tree information.

This needs to be used together with r8a7791 support
code that ties in the R-Car Gen2 arch timer workarounds.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>