ARM: dts: bcm-cygnus: Update spi clock properties

PL022 binding require two clocks to be defined but broadcom cygnus
platform doesn't comply with bindings and define only one clock.

Update spi clocks and clocks-names property by adding appropriate clock
reference to make it compliant with bindings.

CC: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kuldeep Singh <singh.kuldeep87k@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>