ARM: davinci: AM18x: Add wl1271/wlan support

The wl1271 daughter card for AM18x EVMs is a combo wireless connectivity
add-on card, based on the LS Research TiWi module with Texas
Instruments' wl1271 solution.
It is a 4-wire, 1.8V, embedded SDIO WLAN device with an external IRQ
line and is power-controlled by a GPIO-based fixed regulator.

Add support for the WLAN capabilities of this expansion board.

Signed-off-by: Ido Yariv <ido@wizery.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>