ARM: shmobile: r8a7791: Add PCIEC clock device tree node

This patch adds the device tree clock node for the PCIe Controller

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