wl1271: Initial SDIO implementation

Added initial implementation of SDIO interfacte to the wl1271 driver.
When selected, this adds new module called "wl1271_sdio".

Signed-off-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Reviewed-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>