clk: shmobile: cpg-mssr: Update serial port clock in example

Cfr. commit a9ec81f4

 ("serial: sh-sci: Drop the interface
clock").

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Michael Turquette <mturquette@baylibre.com>