arm64: memory: Make virt_to_pfn() a static inline

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.

Since arm64 is using <asm-generic/memory_model.h> to provide
__phys_to_pfn() we need to move the inclusion of that header
up, so we can resolve the static inline at compile time.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>