clocksource/mtu-nomadik: use apb_pclk

After improving the clock tree we need to make sure the the
MTU timer explicitly grabs and enables it silicon clock
(pclk).

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Mike Turquette <mturquette@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>