clk: vexpress: Use common of_clk_init() function

Use common of_clk_init() function for clock initialization.

Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Tested-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>