mxs: ssp: Pull out the SSP bus to regs conversion

Create function which converts SSP bus number to SSP register pointer.
This functionality is reimplemented multiple times in the code, thus
make one common implementation. Moreover, make it a switch(), since the
SSP ports are not mapped in such nice linear fashion on MX23, therefore
having it a switch will simplify things there.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andy Fleming <afleming@freescale.com>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>