ENGR00288133 arm: imx: correct arm clock usecount for i.MX6Q/DL

ENGR00288133 arm: imx: correct arm clock usecount for i.MX6Q/DL

ARM clock is sourcing from pll1_sw, and pll1_sw can be either
from pll1_sys or step, so we should enable arm clock during
clock initialization instead of pll1_sys, otherwise, arm clock's
usecount would be incorrect and PLL1 will never be disabled even
it is not used.

Signed-off-by: Anson Huang <b20788@freescale.com>