mvebu: ds414: Implement Synology specific command set

Synology keeps per item configuration in a dedicated 'partition' in SPI
flash, namely the one named 'vendor' in DTS file. It contains the two
NICs MAC addresses as well as the item's serial number. I didn't find a
way to have this information extracted automatically, therefore
implemented 'syno populate_env' command which extracts the three values
and puts them into environment. To make things permanent though, one has
to 'saveenv'.

Another command is 'syno clk_gate', which allows to change the clock
gating which is done in DS414 board file.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Stefan Roese <sr@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>