mx6: soc: Add the required LDO ramp up delay

When changing LDO voltages we need to wait for the required amount of time
for the voltage to settle.

Also, as the timer is still not available when arch_cpu_init() is called, we
need to call it later at board_postclk_init() phase.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>