Since the addition of the spi reference two styles are used. Use
references instead of recreating the same structure over and over again.

This helps to distinguish which are changes to the underlying nodes and
which are new additions and helps maintainability in general.

Verified the resulting dtb to be binary identical.
Signed-off-by: NRalph Sennhauser <ralph.sennhauser@gmail.com>
Signed-off-by: NImre Kaloz <kaloz@openwrt.org>
Signed-off-by: NGregory CLEMENT <gregory.clement@free-electrons.com>