sparc64: Use HW supported number of context ID bits

sparc64: Use HW supported number of context ID bits

Orabug: 24449941

Number of context IDs supported by the hardware
is reported via machine descriptor for sun4v
systems. For systems > T3, 16 bits are used
to represent context ID in the HW. For these
systems the context ID wrap around happens if
there are more that 65536 processes running
simultaneously. For systems older than that
13 bits are used and the context ID wraps around
if there are 8192 processes running simultaneously.

Reviewed-by: Babu Moger <babu.moger@oracle.com>
Acked-by: Rob Gardner <rob.gardner@oracle.com>
Signed-off-by: Sanath Kumar <sanath.s.kumar@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>