ARM: dts: orange-pi-zero: add node for SPI NOR

Add node for SPI NOR flash on orange-pi-zero board. Disable this node
by default and leave it to users to enable it if their board has
SPI NOR flash chip populated.

SPI NOR flash was optional in the first production batch in Dec 2016.
In later batches flash chip was pre-populated. However there should
be quite a few boards around which do not have flash chip.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>