openrisc: Make pfn accessors statics 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, do the same with pfn_to_virt().

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Stafford Horne <shorne@gmail.com>