clk: rockchip: Fix -Wunused-const-variable in rv1108 clk driver

Clang produces the following warning

drivers/clk/rockchip/clk-rv1108.c:125:7: warning: unused variable
'mux_pll_src_3plls_p' [-Wunused-const-variable]
PNAME(mux_pll_src_3plls_p)      = { "apll", "gpll", "dpll" };

Looks like this variable was never used. Deleting it to remove the
warning.

Cc: clang-built-linux@googlegroups.com
Link: https://github.com/ClangBuiltLinux/linux/issues/524


Signed-off-by: Nathan Huckleberry <nhuck@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>