The carambola2 exposes all the gpio pins, but some are reserved for
core functions (eg usb, ethernet, etc.) Others are configured by default
to be available as normal GPIO pins to do interesting things with.
GPIO 18->23 is the I2S, SLIC and SPDIF device pins, but none of those
are currently used. So, just allow those to show up.
Tested:
* AR9344, Carambola 2
* (.. bitbang SPI to an Adafruit LCD via libgpio, because FreeBSD could
do with more shiny output devices that aren't network interfaces.)
TODO:
There are some other pins aren't currently included here, but should be.
The LED pins are for the internal switch inside the AR9344.
* GPIO 0+1 are "LED0 + LED1", but they're tied to high for bootstrapping.
* GPIO 13-17 are "LED2..7", but they're tied (H, L, L, L, H) for bootstrapping.
* GPIO 11 and 12 are UART RTS/CTS or I2S; but GPIO 12 is tied L for bootstrap.
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