mce-intel-*: fix a warning when using FIELD(<num>, NULL)

mce-intel-*: fix a warning when using FIELD(<num>, NULL)

Internally, FIELD() macro checks the size of an array, by
using ARRAY_SIZE. Well, this macro causes a division by zero
if NULL is used, as its type is void, as warned:

	mce-intel-dunnington.c:30:2: note: in expansion of macro ‘FIELD’
	  FIELD(17, NULL),
	  ^~~~~
	ras-mce-handler.h:28:33: warning: division ‘sizeof (void *) / sizeof (void)’ does not compute the number of array elements [-Wsizeof-pointer-div]
	 #define ARRAY_SIZE(x) (sizeof(x)/sizeof(*(x)))
	                                 ^
	bitfield.h:37:51: note: in expansion of macro ‘ARRAY_SIZE’
	 #define FIELD(start_bit, name) { start_bit, name, ARRAY_SIZE(name) }
	                                                   ^~~~~~~~~~

While this warning is harmless, it may prevent seeing more serios
warnings. So, add a FIELD_NULL(<num>) macro to avoid that.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>