siemens,am33x: add draco etamin board

In the draco CPU board family, etamin is a new variant
with bigger flash and more RAM. Due to new flash that
uses larger pages (4K) some changes are necessary because
it impacts the MTD partition layout and the ubi mount
parameters.

Signed-off-by: Samuel Egli <samuel.egli@siemens.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
[trini: Move BOOTDELAY into defconfig, just always be 3 now]
Signed-off-by: Tom Rini <trini@konsulko.com>