PCI: dwc: Adjust Kconfig to allow IMX6 PCIe host on IMX7

The IMX6 PCI-e host driver also supports the IMX7d. However, the
Kconfig dependencies of the driver prevented it from being enabled
unless the kernel was built with both IMX6 and IMX7 support.

It works fine to build with only IMX7 support enabled therefore
adjust the Kconfig entry to allow this configuration.

Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Cc: Andrey Smirnov <andrew.smirnov@gmail.com>