i.MX6: Add ANATOP regulator init

Init the core regulator voltage to 1.2V. This is required for the correct
functioning of the GPU and when the ARM LDO is set to 1.225V. This is a
workaround to fix some memory clock jitter.

Note: This should be but can't be done in the DCD. The bootloader
      prevents access to the ANATOP registers.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
CC: Jason Chen <b02280@freescale.com>
CC: Jason Liu <r64343@freescale.com>
CC: Ranjani Vaidyanathan <ra5478@freescale.com>
CC: Stefano Babic <sbabic@denx.de>
CC: Fabio Estevam <festevam@gmail.com>