mx6: imx-regs: Remove unused 'omux' field from iomux struct

'omux' field is not used anywhere and such layout is not valid for mx6solox.

Instead of adding more ifdef's into the structure, let's simply remove this
unused 'omux' field.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>