CHROMIUM: mkbp: add missing key for UK keyboard

CHROMIUM: mkbp: add missing key for UK keyboard

This adds KEY_BACKSLASH at (row,column) 4,10 for the UK keyboard, for
which key 3,11 is not wired.

It also replaces the hex code for KEY_102ND with the correct symbol
(don't know why I didn't use that in the first place).

BUG=chrome-os-partner:10534
TEST=manually tested with UK keyboard

Change-Id: I59b0f6dbf8924e10a5cf358b73b59c7cb387e64f
Reviewed-on: https://gerrit-int.chromium.org/20374
Tested-by: Luigi Semenzato <semenzato@google.com>
Commit-Ready: Luigi Semenzato <semenzato@google.com>
Reviewed-by: Luigi Semenzato <semenzato@google.com>
Reviewed-by: Sonny Rao <sonnyrao@google.com>