imx: system counter driver for imx7d and mx6ul

Add system counter driver for imx7d and mx6ul
imx7 and imx6ul supports system counter timer as well as
GPT timer (arch/arm/imx-common/timer.c); The default for
imx7 is systemcounter timer.

Signed-off-by: Ye.Li <B37916@freescale.com>
Signed-off-by: Adrian Alonso <aalonso@freescale.com>