clk: vt8500: Staticize vtwm_pll_ops

'vtwm_pll_ops' is local to this file. Make it static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: Mike Turquette <mturquette@linaro.org>