MIPS: ralink: enable PCI support only if driver for mt7621 SoC is selected

MIPS: ralink: enable PCI support only if driver for mt7621 SoC is selected

Some versions of SoC MT7621 have three PCI express hosts. Some boards
make use of those PCI through the staging driver mt7621-pci. Recently
PCI support has been removed from MT7621 Soc kernel configuration due
to a build error. This makes imposible to compile staging driver and
produces a regression for gnubee based boards. Enable support for PCI
again but enable it only if staging mt7621-pci driver is selected.

Fixes: c4d48cf5e2f0 ("MIPS: ralink: deactivate PCI support for SOC_MT7621")
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: john@phrozen.org
Cc: NeilBrown <neil@brown.name>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20191019081233.7337-1-sergio.paracuellos@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>