clk: renesas: r8a779a0: Fix parent of CBFUSA clock

According to Figure 8.1.1 ("Block Diagram of CPG (R-Car V3U-AD)") in the
R-Car V3U Series User's Manual Rev. 0.5, the parent of the CBFUSA clock
is EXTAL.

Fixes: 17bcc803 ("clk: renesas: cpg-mssr: Add support for R-Car V3U")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20201019120614.22149-3-geert+renesas@glider.be