ARM: 6297/1: move U300 timer to dynamic clock lookup

This moves the U300 timer code to look up its clock rate from the
clock framework as is apropriate and also switches it over to use
the generic code for *calc_mult_shift() on clock source and clock
event.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>