ARM: bcmring: convert to sp804 clocksource

bcmring has a set of four sp804 timers incorporated, yet it has its
own copy of the sp804 code.  Convert its clocksource implementation
to the standard sp804 support code.

Cc: Jiandong Zheng <jdzheng@broadcom.com>
Cc: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>