mfd: stmpe: Probe sub-function by compatible

Use sub-function of_compatible during probe, instead of using the node
name. The code should not rely on the node names during probe, in
addition to that the previously hard-coded node names are not compliant
to the latest naming convention (they are not generic and they use
underscores), and it was broken by mistake already once [1].

[1] commit 56086b5e ("ARM: dts: imx6qdl-apalis: Avoid underscore in node name")

Suggested-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220712163345.445811-3-francesco.dolcini@toradex.com