mfd: stmpe: Support disabling sub-functions

Add support of sub-functions disabling. It allows one to define
an stmpe sub-function device in devicetree, but keep it disabled.

Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211027082155.206449-1-francesco.dolcini@toradex.com