mfd: Allow for const stmpe keyboard data

Since it's not like we will re-arrange the keys at run-time, it
seems proper to allow the keymap data to be const. This solves
a compilation warning in ux500.

Cc: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>