ARM: shmobile: Use clocksource_of_init() on r8a7790

Replace the call to shmobile_timer_init() with
clocksource_of_init(). This will allow us to
get rid of shmobile_timer_init().

Signed-off-by: Magnus Damm <damm@opensource.se>
[horms+renesas@verge.net.au: include linux/clocksource.h]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>