Calculate and update max speed from bus clock for SoCs
using DSPI IP.

The bus clock factor's are taken from the data sheets
of respective SoCs.
Signed-off-by: NBhuvanchandra DV <bhuvanchandra.dv@toradex.com>
Acked-by: NStefan Agner <stefan@agner.ch>
Signed-off-by: NMark Brown <broonie@kernel.org>