mfd: stmpe: Add STMPE_IDX_SYS_CTRL/2 enum

As STMPE1801/1601/24xx has a SYS_CTRL register and
STMPE1601/2403 has even a SYS_CTRL2 register, add
STMPE_IDX_SYS_CTRL/2 and update driver code accordingly

This update prepares the ground for not yet supported STMPE1600
which share similar REG_SYS_CTRL register.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>