ARM: imx: improve the comment of CCM lpm SW workaround

Improve the comment of SW workaround for CCM lpm issue using
hardware errata description to avoid confusion.

ERR007265: CCM: When improper low-power sequence is used, the SoC
enters low power mode before the ARM core executes WFI.

Software workaround:
1) Software should trigger IRQ #32 (IOMUX) to be always pending
   by setting IOMUX_GPR1_GINT.
2) Software should then unmask IRQ #32 in GPC before setting CCM
   Low-Power mode.
3) Software should mask IRQ #32 right after CCM Low-Power mode is
   set (set bits 0-1 of CCM_CLPCR).

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>