timer: imx-gpt: Add timer support for i.MX SoCs family

timer: imx-gpt: Add timer support for i.MX SoCs family

This timer driver uses GPT Timer (General Purpose Timer) available on
a lot of i.MX SoCs family. This driver deals with both 24Mhz oscillator
as well as peripheral clock.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[Giulio: added the driver's stub and handled peripheral clock prescaler
setting making driver to work correctly]
Signed-off-by: Jesse Taube <mr.bossman075@gmail.com>
[Jesse: added init, setting prescaler for 24Mhz support and enabling
timer]