spi: bcm53xx: set of_node to let DT specify device(s)

Setting of_node of master's dev seems to be a common way of letting it
work nicely with DT. This allows specifying device there instead of
hardcoding one in the driver code.

This was successfully tested with commit 1b47b98a ("ARM: BCM5301X:
Add DT entry for SPI controller and NOR flash")

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Mark Brown <broonie@kernel.org>