MIPS: mips-cm: Fix sparse warnings

Sparse emits a bunch of warnings in mips-cm.h due to casting away of
__iomem by the addr_gcr_*() functions:

arch/mips/include/asm/mips-cm.h:134:1: warning: cast removes address space of expression

And subsequent passing of the return values to __raw_readl() and
__raw_writel() in the read_gcr_*() and write_gcr_*() functions:

arch/mips/include/asm/mips-cm.h:134:1: warning: incorrect type in argument 2 (different address spaces)
arch/mips/include/asm/mips-cm.h:134:1:    expected void volatile [noderef] <asn:2>*mem
arch/mips/include/asm/mips-cm.h:134:1:    got unsigned int [usertype] *
arch/mips/include/asm/mips-cm.h:134:1: warning: incorrect type in argument 1 (different address spaces)
arch/mips/include/asm/mips-cm.h:134:1:    expected void const volatile [noderef] <asn:2>*mem
arch/mips/include/asm/mips-cm.h:134:1:    got unsigned int [usertype] *

Fix by adding __iomem to the addr_gcr_*() return type and cast.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8874/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>