clk: kendryte/pll.h: do not redefine nop()

The kendryte PLL code uses nop as barrier. The macro is not defined for
the sandbox on x86 but is defined on RISC-V.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>