clocksource/drivers/lpc32xx: Don't use the prescaler counter for clockevents

This commit switches the clockevents one-shot current implementation
to avoid using the prescaler counter. The clockevents timer currently
uses MR0=1, PR=ticks; and after this commit is uses MR0=ticks, PR=0.

While using the prescaler with PR=1 works fine in one-shot mode,
it seems it doesn't work as expected in periodic mode.

By using the only match channel register (MR0) for the timer we make
the periodic mode introduction easier, and consistent with one-shot mode.

Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Joachim Eastwood <manabian@gmail.com>
Tested-by: Joachim Eastwood <manabian@gmail.com>