input: touchscreen: st: enable manual firmware upgrade

Write fts_fwupdate node to enable manual firmware upgrade, flash
the firmware file directly and won't check if the firmware is new.

Change-Id: I9dd768dafbac9f4e3011193bb9de4ac18b04a31d
Signed-off-by: Fei Mao <feim1@codeaurora.org>