clk: mmp: avoid missing prototype warning

The kernel test robot points out two harmless warnings in the
mmp clk drivers:

drivers/clk/mmp/clk-pxa168.c:68:13: warning: no previous prototype for 'pxa168_clk_init' [-Wmissing-prototypes]
drivers/clk/mmp/clk-pxa910.c:66:13: warning: no previous prototype for 'pxa910_clk_init' [-Wmissing-prototypes]

Fix these by including corresponding header file.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200729113456.4072290-1-arnd@arndb.de


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