ARM: shmobile: rcar-gen2: Add CA7 arch_timer initialization for r8a7794

On E2, the arch timer is hooked up to a different clock, and the CA7's arch
timer CNTVOFF register must be initialized.

Based on work by Hisashi Nakamura.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>