powerpc: Fix unterminated of_device_id array in legacy_serial.c

A recent patch to legacy_serial.c factored out some code by
using the of_match_node() facility to match a node against
an array of possible matches. However, the patch didn't properly
terminate the array causing potential crashes in cases where no
match is found. In addition, the name of the array was poorly
chosen for a static symbol making debugging harder.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>