clk: renesas: r8a7796: Add INTC-EX clock

Add the "intc-ex" clock to the R8A7796 CPG MSSR driver.

According to information from the hardware team the INTC-EX
parent clock is CP. The next data sheet version will include
this information.

[takeshi.kihara.df: Ported from commit f099aa07 ("clk: shmobile:
 r8a7795: Add INTC-EX clock") to drivers/clk/renesas/r8a7796-cpg-mssr.c]
Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>