to the following behavior:

 - when setting GPIOs, ignore the higher order bits (they're for
   clearing, we don't need to care about them).

 - when clearing GPIOs, keep all the bits, but unset (via XOR) the
   lower order bit that negates the clear bit that we care about.  That
   is, if we're clearing GPIO 26 (val = 0x04000000), we first XOR what's
   currently in the register with 0x0400 (GPIO 26's SET bit), and then
   OR that with the GPIO 26's CLEAR bit.

Tested-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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