board: colibri_imx7: fix usb start on solo

This fixes the following crash when run on a Colibri iMX7S aka solo:

Colibri iMX7 # usb start
starting USB...
Bus usb@30b10000: USB EHCI 1.00
Bus usb@30b20000:

The i.MX 7Solo has only one single USB OTG1 but no USB host port. Trying
to initialize the nonexisting port just crashes.

While at it also drop board_usb_phy_mode() which is also no longer used
in the driver model age.

Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
diff --git a/configs/colibri_imx7_defconfig b/configs/colibri_imx7_defconfig
index 2f61223..0e254fd 100644
--- a/configs/colibri_imx7_defconfig
+++ b/configs/colibri_imx7_defconfig
@@ -10,6 +10,7 @@
 CONFIG_IMX_RDC=y
 CONFIG_IMX_BOOTAUX=y
 CONFIG_IMX_HAB=y
+CONFIG_OF_BOARD_FIXUP=y
 CONFIG_SYS_MEMTEST_START=0x80000000
 CONFIG_SYS_MEMTEST_END=0x8c000000
 CONFIG_DISTRO_DEFAULTS=y