ARM: imx: define an enum for gpt timer device type

Define an enum for gpt timer device type in include/soc/imx/timer.h to
tell the gpt block differences among SoCs.  Update non-DT users (clock
drivers) to pass the device type.

As we now have include/soc/imx/timer.h, the declaration of
mxc_timer_init() is moved into there as the best fit.

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