mx6: Remove duplication of iomuxc structure

There is no need to keep iomuxc_base_regs structure as it serves the exact same
purpose of the iomuxc structure, which is to provide access to the GPR
registers.

The additional fields of iomuxc_base_regs are not used. Other advantage of
'iomuxc' is that it has a shorter name and the variable declarations can fit
into a single line.

So remove iomuxc_base_regs structure and use iomuxc instead.

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