ARM: shmobile: force enable of r8a7790 arch timer

Implement a SoC-specific timer init function for
r8a7790 that makes sure the architected timer
is started regardless of boot loader setting.

Signed-off-by: Magnus Damm <damm@opensource.se>
Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>