microblaze: Fix pci compilation and sparse warnings

microblaze: Fix pci compilation and sparse warnings

Warning log:
  CHECK   arch/microblaze/pci/pci-common.c
arch/microblaze/pci/pci-common.c:290:14: warning: Using plain integer as NULL pointer
arch/microblaze/pci/pci-common.c:1127:6: warning: symbol
'pcibios_allocate_bus_resources' was not declared. Should it be static?
arch/microblaze/pci/pci-common.c:1436:61: warning: incorrect type in argument 3 (different base types)
arch/microblaze/pci/pci-common.c:1436:61:    expected unsigned int [unsigned] [usertype] offset
arch/microblaze/pci/pci-common.c:1436:61:    got void [noderef] <asn:2>*
  CC      arch/microblaze/pci/pci-common.o
arch/microblaze/pci/pci-common.c: In function 'pci_proc_domain':
arch/microblaze/pci/pci-common.c:825:25: warning: unused variable 'hose' [-Wunused-variable]
arch/microblaze/pci/pci-common.c: In function 'pcibios_allocate_bus_resources':
arch/microblaze/pci/pci-common.c:1182:1: warning: label 'clear_resource' defined but not used [-Wunused-label]
arch/microblaze/pci/pci-common.c: In function 'pcibios_setup_phb_resources':
arch/microblaze/pci/pci-common.c:1436:2: warning: passing argument 3 of
'pci_add_resource_offset' makes integer from pointer without a cast [enabled by default]
include/linux/pci.h:999:6: note: expected 'resource_size_t' but argument is of type 'void *'

Signed-off-by: Michal Simek <michal.simek@xilinx.com>