clock: redefine variable clocks_per_pll as a struct member

redefine variable clocks_per_pll as a struct member

If there are multiple PLL clock nodes, this variable will
get overwritten. Redefining it as a struct member can avoid that.

Signed-off-by: Tang Yuantian <Yuantian.Tang@freescale.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>