mx6sabresd: Convert PCI to driver model

Convert imx6sabresd PCI to driver model to fix the following warning:

===================== WARNING ======================
This board does not use CONFIG_DM_PCI Please update
the board to use CONFIG_DM_PCI before the v2019.07 release.
Failure to update by the deadline may result in board removal.
See doc/driver-model/MIGRATION.txt for more info.
====================================================

After the conversion the following commands were used for testing:

=> pci enum
PCI: Failed autoconfig bar 10
PCI: Failed autoconfig bar 10
=> pci 1
Scanning PCI devices on bus 1
BusDevFun  VendorId   DeviceId   Device Class       Sub-Class
Reviewed-by: Fabio Estevam <festevam@gmail.com>
diff --git a/configs/mx6sabresd_defconfig b/configs/mx6sabresd_defconfig
index c6cd1e3..8629554 100644
--- a/configs/mx6sabresd_defconfig
+++ b/configs/mx6sabresd_defconfig
@@ -83,6 +83,7 @@
 CONFIG_PHYLIB=y
 CONFIG_MII=y
 CONFIG_PCI=y
+CONFIG_DM_PCI=y
 CONFIG_PINCTRL=y
 CONFIG_PINCTRL_IMX6=y
 CONFIG_DM_REGULATOR=y