imx: ventana: split out common functions between SPL and uboot

Move shared functions used by both SPL and U-Boot to common.c:
 - setup_iomux_uart() and uart pad config
 - gpio pad config

In the process also moved the following to common.c in preparation for
calling it from the SPL:
 - split i2c setup into a shared function
 - move pmic init to setup_pmic() function to call directly from
   power_init_board()
 - split gpio setup into early (iomux and default pin config)
   and late (output configuration based on env)

Signed-off-by: Tim Harvey <tharvey@gateworks.com>