PCI: keystone: Cleanup macros defined in pci-keystone.c

No functional change. Cleanup macros defined in pci-keystone.c
by removing unused macros, grouping the macros and aligning
it properly.

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