ARM: u300: switch SSP/SPI clock name to "SSPCLK"

As noted in recent discussions the name of the core clock for
the PL022 derived SPI blocks is erroneously named in the
U300 device tree. The kernel doesn't currently use the name,
but may do so soon so let use rename all these clocks in
accordance with the name given in the PL022 TRM (ARM DDI 0194G).

Reviewed-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>