ARM: imx: clk: correct arm clock usecount

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>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>