mfd: stmpe: Fix bit clearing on STMPE1600

GPIO bits clearing on pins assigned to STMPE1600
had no effects due to missing "clear registers"
settings within stmpe1600_regs[].
STMPE1600 does not have dedicated "clear registers",
but single "set/clear registers", hence stmpe1600_regs[]
"clear registers" (STMPE_IDX_GPCR_XXX) must be set to
same value as "set registers" (STMPE_IDX_GPSR_XXX), ie
STMPE1600_REG_GPSR_XXX.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>