clk: ux500: Add a 2-cell Device Tree parser for obtaining PRCC clocks

PRCC (peripheral and kernel) clocks are specified using a property tuple
<&phandle base bit>, where 'base' is the peripheral (1, 2, 3, 5 or 6),
and bit is read-in value into that peripheral stipulated by the hardware
specification.

Acked-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>