mcx: support for HTKW mcx board

This patch adds support for the HTKW mcx AM3517-based board.
Serial, Ethernet, NAND, MMC, RTC, EHCI USB host and both
NAND and MMC SPLs are supported.

Signed-off-by: Ilya Yanok <yanok@emcraft.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Tom Rini <tom.rini@gmail.com>
Cc: Detlev Zundel <dzu@denx.de>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>