powerpc/83xx: Fix sparse warnings in board files

This patch fixes following sparse warnings:

  CHECK   83xx/usb.c
83xx/usb.c:205:5: warning: symbol 'mpc837x_usb_cfg' was not declared. Should it be static?
  CHECK   83xx/mpc831x_rdb.c
83xx/mpc831x_rdb.c:45:13: warning: symbol 'mpc831x_rdb_init_IRQ' was not declared. Should it be static?
  CHECK   83xx/mpc832x_rdb.c
83xx/mpc832x_rdb.c:133:13: warning: symbol 'mpc832x_rdb_init_IRQ' was not declared. Should it be static?
  CHECK   83xx/mpc832x_mds.c
83xx/mpc832x_mds.c:68:12: warning: Using plain integer as NULL pointer
83xx/mpc832x_mds.c:72:13: warning: incorrect type in assignment (different address spaces)
83xx/mpc832x_mds.c:72:13:    expected unsigned char [usertype] *static [toplevel] bcsr_regs
83xx/mpc832x_mds.c:72:13:    got void [noderef] <asn:2>*
83xx/mpc832x_mds.c:99:11: warning: incorrect type in argument 1 (different address spaces)
83xx/mpc832x_mds.c:99:11:    expected void volatile [noderef] <asn:2>*addr
83xx/mpc832x_mds.c:99:11:    got unsigned char [usertype] *static [toplevel] bcsr_regs

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>