Allow manual bed leveling inputs

CR6Community/Marlin#209

VP list generated using:

 $Start = 0x2360;$Current=$Start;for ($X = 1; $X -le 4; $X++) {
>> "COL $X"
>> for ($Y = 1; $Y -le 4;$Y++) {
>> "($X,$Y) {0:x2} - {1}" -f $Current,$Point
>> $Current += $Offset
>> $Point++
>> }
>> }

Output:

COL 1
(1,1) 2360 - 48
(1,2) 2362 - 49
(1,3) 2364 - 50
(1,4) 2366 - 51
COL 2
(2,1) 2368 - 52
(2,2) 236a - 53
(2,3) 236c - 54
(2,4) 236e - 55
COL 3
(3,1) 2370 - 56
(3,2) 2372 - 57
(3,3) 2374 - 58
(3,4) 2376 - 59
COL 4
(4,1) 2378 - 60
(4,2) 237a - 61
(4,3) 237c - 62
(4,4) 237e - 63