ARM: 6947/2: mach-u300: fix compilation error in timer

The introduction of the mmio timer accidentally referenced the
old clocksource struct which does not exist anymore. Fix this by
using a simple string instead.

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