clk: imx8mn: add GPT support

Add support for the General Purpose Timer (GPT) clocks on the i.MX8MN.
The i.MX8MN GPT IP block is the same as on the i.MX8MM, on which this
patch is based.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Link: https://lore.kernel.org/r/20220317223600.175894-2-alvin@pqrs.dk


Signed-off-by: Abel Vesa <abel.vesa@nxp.com>