dt-bindings: imx: add clock bindings for i.MX8MN GPT

The i.MX8MN has a General Purpose Timer (GPT) just like the i.MX8MM,
which already has such bindings. Add the relevant bindings for the Nano
SoC too.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Link: https://lore.kernel.org/r/20220317223600.175894-1-alvin@pqrs.dk
Signed-off-by: Abel Vesa <abel.vesa@nxp.com>