arm: juno: Enable PCI

The ARM Juno boards in their -r1 and -r2 variants sport a PCIe
controller, which we configure already in board specific code to be ECAM
compliant. Hence we can just enable the generic ECAM driver to let
U-Boot use PCIe devices.

Add the respective options to the Juno defconfig to enable the PCI
framework and the generic ECAM driver, and initialise the driver upon
loading U-Boot.

Make some functions in the Juno PCIe init code static on the way.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>