clk: mmp2: fix link error without mmp2

The newly added function is only built into the kernel if mmp2
is enabled, causing a link error otherwise.

arm-linux-gnueabi-ld: drivers/clk/mmp/clk.o: in function `mmp_register_pll_clks':
clk.c:(.text+0x6dc): undefined reference to `mmp_clk_register_pll'

Move it to a different file to get it to link.

Fixes: 5d34d0b3

 ("clk: mmp2: Add support for PLL clock sources")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/20200408160518.2798571-1-arnd@arndb.de
Reported-by: Guenter Roeck <linux@roeck-us.net>
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>