When building for Cyclone5 SoCFPGA, the socfpga_a10_pll_init(),
socfpga_a10_perith_init() and socfpga_a10_gate_init() functions are
defined as dummy functions returning ERR_PTR(-ENOSYS).  They are defined
with external linkage.  With '-Wmissing-prototypes' GCC warns about
externally linked function definitions with no preceding prototype.
Define them as 'static inline' to avoid the compiler warnings.

(Note: Arria10 uses non-dummy versions of these functions declared
'extern' but defined elsewhere.)

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>