mx6: soc: Clear the LDO ramp values up prior to setting the LDO voltages

mx6: soc: Clear the LDO ramp values up prior to setting the LDO voltages

Since ROM may modify the LDO ramp up time according to fuse setting,
it is safer to reset the ramp up field to its default value of 00:

00: 64 cycles of 24MHz clock;
01: 128 cycles of 24MHz clock;
02: 256 cycles of 24MHz clock;
03: 512 cycles of 24MHz clock;

Signed-off-by: Anson Huang <b20788@freescale.com>
Signed-off-by: Jason Liu <r64343@freescale.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>