PCI: keystone: Don't select CONFIG_PCI_KEYSTONE_HOST by default

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

Remove the "default y" for CONFIG_PCI_KEYSTONE_HOST.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: linux-arm-kernel@lists.infradead.org