mx23evk: Add initial board support

The following features are supported:
 * 128 MB DDR1 SDRAM
 * DUART
 * SD/MMC Card Socket

Series-to: uboot
Series-version: 5
Series-changes:
- Use 4bit bus for mmc

Series-changes: 4
- Fix build warnings

Series-changes: 3
- Removed unused macro for GPMI setting in spl_boot.c

Series-changes: 2
- Add DUART in commitlog
- Remove AUART MUX setup as it is unused
- Remove wrong comment in mx23evk.h board config
- Use single line comments in mx23evk.h board config
- Use '=>' as prompt
- Remove SPI, RTC and USB support as those where not tested yet

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>