clk: renesas: r8a779f0: Correct PFC/GPIO parent clock

According to the R-Car S4 Series Hardware User’s Manual Rev.0.81, the
parent clock of the Pin Function (PFC/GPIO) module clock is the CP
clock.

As this clock is not documented to exist on R-Car S4, use the CPEX clock
instead.

Fixes: 73421f2a ("clk: renesas: r8a779f0: Add PFC clock")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f88ec4aede0eaf0107c8bb7b28ba719ac6cd418f.1706197415.git.geert+renesas@glider.be