ARM: mach-shmobile: convert ag5evm to use the generic MMC GPIO hotplug helper

This also fixes the following modular mmc build failure:

arch/arm/mach-shmobile/built-in.o: In function `mackerel_sdhi0_gpio_cd':
pfc-sh7372.c:(.text+0x1138): undefined reference to `mmc_detect_change'

on this platform by eliminating the use of an inline function, which
calls into the mmc core.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Simon Horman <horms@verge.net.au>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>