ARM: dts: r8a7793: Correct parent of SSI[0-9] clocks

The SSI-ALL gate clock is located in between the P clock and the
individual SSI[0-9] clocks, hence the former should be listed as their
parent.

Fixes: 072d3265 ("ARM: dts: r8a7793: add MSTP10 clocks to device tree")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>