MMC: OMAP: Make board-sx1.c uses new board-sx1-mmc.c code

Make board-sx1.c file uses new board-sx1-mmc.c code.

This patch also:
- Introduces a new gpio_switch to get events when inserting and removing
  MMC/SD/SDIO cards to/from the slot.
- Enables the GPIO switch support option on sx1_defconfig to use
  multislot support:

Signed-off-by: Carlos Eduardo Aguiar <carlos.aguiar@indt.org.br>
Signed-off-by: Tony Lindgren <tony@atomide.com>