powerpc/83xx: Fix sparse warnings in mpc836x_mds.c

This patch fixes following sparse warnings:

  CHECK   mpc836x_mds.c
mpc836x_mds.c:75:12: warning: Using plain integer as NULL pointer
mpc836x_mds.c:79:13: warning: incorrect type in assignment (different address spaces)
mpc836x_mds.c:79:13:    expected unsigned char [usertype] *static [toplevel] bcsr_regs
mpc836x_mds.c:79:13:    got void [noderef] <asn:2>*
mpc836x_mds.c:105:3: warning: incorrect type in argument 1 (different address spaces)
mpc836x_mds.c:105:3:    expected unsigned char volatile [noderef] [usertype] <asn:2>*addr
mpc836x_mds.c:105:3:    got unsigned char [usertype] *
mpc836x_mds.c:105:3: warning: incorrect type in argument 1 (different address spaces)
mpc836x_mds.c:105:3:    expected unsigned char const volatile [noderef] [usertype] <asn:2>*addr
mpc836x_mds.c:105:3:    got unsigned char [usertype] *
mpc836x_mds.c:107:3: warning: incorrect type in argument 1 (different address spaces)
mpc836x_mds.c:107:3:    expected unsigned char volatile [noderef] [usertype] <asn:2>*addr
mpc836x_mds.c:107:3:    got unsigned char [usertype] *
mpc836x_mds.c:107:3: warning: incorrect type in argument 1 (different address spaces)
mpc836x_mds.c:107:3:    expected unsigned char const volatile [noderef] [usertype] <asn:2>*addr
mpc836x_mds.c:107:3:    got unsigned char [usertype] *
mpc836x_mds.c:131:11: warning: incorrect type in argument 1 (different address spaces)
mpc836x_mds.c:131:11:    expected void volatile [noderef] <asn:2>*addr
mpc836x_mds.c:131:11:    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>