pinctrl: pinmux: add function selector to pinmux-functions

pinctrl: pinmux: add function selector to pinmux-functions

Add the function selector to the pinmux-functions debugfs output. This
is an integer which is the index into the pinmux function tree.  It will
make it easier to correlate function name to function selector without
having to count the lines in the output.

Example output of "pinmux-functions":

function 0: pinmux-uart0-pins, groups = [ pinmux-uart0-pins ]
function 1: pinmux-uart1-pins, groups = [ pinmux-uart1-pins ]
function 2: pinmux-uart2-pins, groups = [ pinmux-uart2-pins ]
function 3: pinmux-mmc0-pins, groups = [ pinmux-mmc0-pins ]
function 3: pinmux-mmc1-pins, groups = [ pinmux-mmc1-pins ]
function 5: pinmux-i2c0-pins, groups = [ pinmux-i2c0-pins ]
function 6: pinmux-i2c1-pins, groups = [ pinmux-i2c1-pins ]
function 7: pinmux-i2c2-pins, groups = [ pinmux-i2c2-pins ]
function 8: pinmux-pwm0-pins, groups = [ pinmux-pwm0-pins ]
function 9: pinmux-pwm1-pins, groups = [ pinmux-pwm1-pins ]
function 10: pinmux-adc-pins, groups = [ pinmux-adc-pins ]

Cc: Jason Kridner <jkridner@beagleboard.org>
Cc: Robert Nelson <robertcnelson@beagleboard.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Drew Fustini <drew@beagleboard.org>
Link: https://lore.kernel.org/r/20210123202212.528046-1-drew@beagleboard.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>