arm64: dts: ns2: Fix spi clock name

SPI clock name for pl022 is "sspclk" and not "spiclk".
Also fix below dtc warning:
clock-names:0: 'spiclk' is not one of ['SSPCLK', 'sspclk']

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