clk: mmp2: Remove a unused prototype

There is no mmp_clk_register_pll2() routine.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lkml.kernel.org/r/20200309194254.29009-2-lkundrak@v3.sk


Signed-off-by: Stephen Boyd <sboyd@kernel.org>