PCI: keystone: Explicitly set the PCIe mode

Explicitly set the PCIe mode to BOOTCFG_DEVCFG instead of always
relying on the default values. This is required when EP mode has to
be explicitly written to BOOTCFG_DEVCFG register.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>