Input: stmpe-ts - fix automatic module loading

When STMPE is instantiated via device tree individual MFD cells rae formed
with OF modaliases, not platform modaliases, and so we need to add OF
device table to the driver if we want it to load automatically:

of:Nstmpe_touchscreenT<NULL>Cst,stmpe-ts

Reported-by: Heiner Kallweit <hkallweit1@gmail.com>
Tested-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>