ARM: imx_v6_v7_defconfig: Fix unintentional disablement of PCI

Since commit 75c2f26d ("PCI: imx6: Add i.MX PCIe EP mode support")
the i.MX6 PCI driver is no longer selected by default. The
existing PCI_IMX6 was made a hidden option, selected by new options
PCI_IMX6_HOST (for the existing support) and PCI_IMX6_EP (for the
endpoint mode), but there has been no corresponding update to
imx_v6_v7_defconfig so the PCI_IMX6 ends up getting disabled.
Switch imx_v6_v7_defconfig to PCI_IMX6_HOST to preserve the existing
functionality.

This is based on the same fix done in commit 0cd5780e ("arm64:
defconfig: Fix unintentional disablement of PCI on i.MX").

Fixes: 75c2f26d ("PCI: imx6: Add i.MX PCIe EP mode support")
Reported-by: Mattias Barthel <mattiasbarthel@gmail.com>
Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>