x86, UV: Add common uv_early_read_mmr() function for reading MMRs

Early in boot, reading MMRs from the UV hub controller require
calls to early_ioremap()/early_iounmap().  Rather than
duplicating code, add a common function to do the
map/read/unmap.

Signed-off-by: Jack Steiner <steiner@sgi.com>
LKML-Reference: <20101130195926.834804371@sgi.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>