perf/x86/intel/uncore: Factor out __snr_uncore_mmio_init_box

The IMC uncore unit in Ice Lake server can only be accessed by MMIO,
which is similar as Snow Ridge.
Factor out __snr_uncore_mmio_init_box which can be shared with Ice Lake
server in the following patch.

No functional changes.

Signed-off-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/1584470314-46657-2-git-send-email-kan.liang@linux.intel.com