dt/documentation: Fix value format description

Numeric values in dts files can be specified in decimal and hex (the latter
prefixed 0x). The current documentation is updated with this patch to prevent
confusion about what is meant with values without "0x" (previously hex, now
dec).

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Rob Landley <rob@landley.net>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>