MIPS: Fix GCC-4.2 'discards qualifiers from pointer target type' warnings

MIPS: Fix GCC-4.2 'discards qualifiers from pointer target type' warnings

Compiling dbau1x00 and gth2 boards with GCC-4.2, you would see new warnings
like this:

skuribay@ubuntu:u-boot.git$ ./MAKEALL dbau1000
Configuring for dbau1x00 board...
au1x00_eth.c: In function 'au1x00_send':
au1x00_eth.c:158: warning: passing argument 1 of 'virt_to_phys' discards qualifiers from pointer target type
au1x00_eth.c: In function 'au1x00_recv':
au1x00_eth.c:211: warning: passing argument 1 of 'virt_to_phys' discards qualifiers from pointer target type
au1x00_eth.c: In function 'au1x00_init':
au1x00_eth.c:252: warning: passing argument 1 of 'virt_to_phys' discards qualifiers from pointer target type
au1x00_eth.c: In function 'au1x00_recv':
au1x00_eth.c:211: warning: passing argument 1 of 'virt_to_phys' discards qualifiers from pointer target type
au1x00_eth.c: In function 'au1x00_init':
au1x00_eth.c:252: warning: passing argument 1 of 'virt_to_phys' discards qualifiers from pointer target type
au1x00_eth.c: In function 'au1x00_send':
au1x00_eth.c:158: warning: passing argument 1 of 'virt_to_phys' discards qualifiers from pointer target type

We're passing a volatile pointer to a function which is expecting a non-
volatile pointer.  That's potentially dangerous, so gcc warns about it.
Confirmed with ELDK 4.2 (GCC 4.2.2) and Sourcey G++ 4.2 (GCC 4.2.3).

To fix this, we add a volatile attribute to the argument in question.
The virt_to_phys function in Linux kernel also does the same thing.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>