[ARM] 2893/1: [MMC] Update corgi to use the new mmc delayed detection function

Patch from Richard Purdie

We can remove this timer and its associated code from the corgi
platform code now mmc_detect_change() and the pxamci code support
an optional delay.

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>