arm: mx23: Fix VDDMEM misconfiguration

arm: mx23: Fix VDDMEM misconfiguration

The VDDMEM ramped up in very weird way as it was horribly misconfigured.
Instead of setting up VDDMEM in one swipe, let it rise slowly the same
way as VDDD and VDDA in spl_power_init.c and then only clear ILIMIT before
memory gets inited. This makes sure the VDDMEM rises sanely, not jumps up
and down as it did till now.

The VDDMEM prior to this change did this:
     2V0____   .--------2V5
       |    `--'
 0V____|

The VDDMEM now does this:
    2V0_____,-----------2V5
      /
 0V__|

Moreover, VDDIO on MX23 uses 25mV steps while MX28 uses 50mV steps,
fix this difference too.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>