The idea is to give useful names to GPIO lines that an implementer
will be using from userspace, e.g. for maker type projects.  These are
user-visible using tools/gpio/lsgpio.c

v2: Major rewrite by anholt: Flatten each GPIO line to a line in the
    file for better diffing, prefix all expansion header pins with
    "P<number>" or "P5HEADER_P<number>" and drop the mostly-unused
    GPIO_GEN<smallnumber> names in favor of GPIO<socgpionumber>, fix
    extra '[]' on a couple of lines, fix locations of SD_CARD_DETECT,
    CAM_GPIO and STATUS_LED, fix HDMI_HPD polarities, rewrite A+ using
    unreleased schematics.

v3: More changes by anholt: Drop P<number> / P5HEADER<number>
    prefixes.  I had been skeptical about adding them, and was
    convinced to drop them by Gottfried (who probably has more
    experience with GPIOs in educational contexts than the rest of
    us).  Also drop [] brackets for "is pinmuxed", which didn't seem
    to clarify, and were ambiguous for things like the SPI_*-labeled
    pins which may or may not actually be pinmuxed to SPI.

v4: Rename B+'s SDA0/SCL0 to match the other boards, despite the
    naming on its schematic.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Eric Anholt <eric@anholt.net>