ARM: dts: r8a7791: 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: ee914152 ("ARM: shmobile: r8a7791: add MSTP10 support on DTSI")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>