asm-generic/page.h: Make pfn accessors static inlines

Making virt_to_pfn() a static inline taking a strongly typed
(const void *) makes the contract of a passing a pointer of that
type to the function explicit and exposes any misuse of the
macro virt_to_pfn() acting polymorphic and accepting many types
such as (void *), (unitptr_t) or (unsigned long) as arguments
without warnings.

For symmetry we do the same change for pfn_to_virt.

Immediately define virt_to_pfn and pfn_to_virt to the static
inline after the static inline since this style of defining
functions is used for the generic helpers.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>