x86/platform/uv: Introduce helper function uv_pnode_to_socket.

Add and use uv_pnode_to_socket() function, which parallels other
helper functions in here, and will enable avoiding duplicate code
in an upcoming patch.

Signed-off-by: Steve Wahl <steve.wahl@hpe.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/all/20230519190752.3297140-3-steve.wahl%40hpe.com