dt-bindings: timer: fsl,imxgpt: Add optional osc_per clock

dt-bindings: timer: fsl,imxgpt: Add optional osc_per clock

Since commit bad3db104f89 ("ARM: imx: source gpt per clk from OSC for
system timer") osc_per can be used for clocking the GPT which is not
scaled when entering low bus mode.
This clock source is available only on i.MX6Q (incl. i.MX6QP) and i.MX6DL.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230810144451.1459985-7-alexander.stein@ew.tq-group.com
Signed-off-by: Rob Herring <robh@kernel.org>