GPIO: Add GPIO_W to read/write individual GPIO pins via words.
Add repetitive interrupt timer (RIT) bit/field #defines.
SDIO: Add register bit/field #defines.
SDIO: Remove UHS_REG, which NXP apparently un-documented.
Surround headers with C extern linkage for C++.
Merge pull request #24 from jboone/master

A few new bits for libopencm3