PCI: dra7xx: Don't select CONFIG_PCI_DRA7XX_HOST by default

Drivers should not be selected by default because that bloats the kernel
for people who don't need them.

Enable CONFIG_PCI_DRA7XX_HOST by default only if SOC_DRA7XX.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: linux-omap@vger.kernel.org