ARM: dts: ste-dbx: Update spi clock-names property

ARM: dts: ste-dbx: Update spi clock-names property

Now that spi pl022 binding only accept "sspclk" as clock name, ST
ericsson platform with "SSPCLK" clock name start raising dtbs_check
warnings. Make necessary changes to update this property in order to
make it compliant with binding.

clock-names:0: 'sspclk' was expected

Signed-off-by: Kuldeep Singh <singh.kuldeep87k@gmail.com>
Link: https://lore.kernel.org/r/20220312113853.63446-5-singh.kuldeep87k@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>