ARM: dts: r8a7794: Correct hsusb parent clock

The parent clock of the HSUSB clock is the HP clock, not the MP clock.

Fixes: c7bab9f9

 ("ARM: shmobile: r8a7794: Add USB clocks to device tree")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>