wl1271: Changed wl1271_sdio to be selectable only on ARM

wl1271: Changed wl1271_sdio to be selectable only on ARM

As wl1271_sdio implementation depends on ARM GPIO impelementation it is
not directly usable on other architectures at the moment. Added
ARM dependency to kernel configuration option.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Reviewed-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>