PCI: keystone: Cleanup outbound window configuration

Outbound translation window is configured in order to access the
PCIe card's MEM space. Cleanup outbound translation configuration
here by using BIT() macros, adding a macro for window size and
using lower_32_bits/upper_32_bits macros for configuring the 64 bit
offset in the outbound translation region.

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