mm: fix type cast in __pfn_to_phys()

The various definitions of __pfn_to_phys() have been consolidated to
use a generic macro in include/asm-generic/memory_model.h. This hit
mainline in the form of 012dcef3 "mm: move __phys_to_pfn and
__pfn_to_phys to asm/generic/memory_model.h". When the generic macro
was implemented the type cast to phys_addr_t was dropped which caused
boot regressions on ARM platforms with more than 4GB of memory and
LPAE enabled.

It was suggested to use PFN_PHYS() defined in include/linux/pfn.h
as provides the correct logic and avoids further duplication.

Reported-by: kernelci.org bot <bot@kernelci.org>
Suggested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>