gpio: stmpe: forbid unused lines to be mapped as IRQs

Exploit the new mechanism for masking off disallowed IRQs
added by Mika Westerberg to properly manage the STMPE
"norequest mask" to disallow also mapping said lines as
IRQs.

Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>