i2c-ocores: Fix pointer type mismatch error

ocores_i2c_of_probe needs to use a const __be32 type for handing
device tree property values.  This patch fixed the following build
warning:

 CC      drivers/i2c/busses/i2c-ocores.o
  drivers/i2c/busses/i2c-ocores.c: In function 'ocores_i2c_of_probe':
  drivers/i2c/busses/i2c-ocores.c:254: warning: assignment discards qualifiers from pointer target type
  drivers/i2c/busses/i2c-ocores.c:261: warning: assignment discards qualifiers from pointer target type

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Peter Korsgaard <jacmet@sunsite.dk>
Cc: Ben Dooks <ben-linux@fluff.org>
Cc: linux-i2c@vger.kernel.org
Signed-off-by: Ben Dooks <ben-linux@fluff.org>