dt-bindings: clock: fsl-sai: Document i.MX8M support

dt-bindings: clock: fsl-sai: Document i.MX8M support

The i.MX8M/Mini/Nano/Plus variant of the SAI IP has control registers
shifted by +8 bytes and requires additional bus clock. Document support
for the i.MX8M variant of the IP with this register shift and additional
clock. Update the description slightly.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Marek Vasut <marex@nabladev.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>