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>
diff --git a/board/gateworks/gw_ventana/Makefile b/board/gateworks/gw_ventana/Makefile
index 33a1788..8fa691a 100644
--- a/board/gateworks/gw_ventana/Makefile
+++ b/board/gateworks/gw_ventana/Makefile
@@ -6,6 +6,6 @@
 # SPDX-License-Identifier:  GPL-2.0+
 #
 
-obj-y  := gw_ventana.o gsc.o eeprom.o
+obj-y  := gw_ventana.o gsc.o eeprom.o common.o
 obj-$(CONFIG_SPL_BUILD) += gw_ventana_spl.o