MIPS: Decode core number for R2 CPUs.

The struct cpuinfo_mips.core field should be populated with the
physical core number.  For R2 CPUs, this is carried in the low 10 bits
of Ebase.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/1505/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>