ARM: shmobile: r8a7791: Add I2C clocks

This adds I2C[0-5] clock support to R8A7791 SoC.

Changes in V2:
* Capitalized ARM in the subject.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>