x86/UV: Set n_lshift based on GAM_GR_CONFIG MMR for UV3

The value of n_lshift for UV is currently set based on the
socket m_val.

For UV3, set the n_lshift value based on the GAM_GR_CONFIG MMR.
This will allow bios to control the n_lshift value independent
of the socket m_val. Then n_lshift can be assigned a fixed value
across a multi-partition system, allowing for a fixed common
global physical address format that is independent of socket
m_val.

Cleanup unneeded macros.

Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
Link: http://lkml.kernel.org/r/20140331143700.GB29916@sgi.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>