CONFIG_PHYS_64BIT is not available for the default architecture x86.
Result is shown for architecture powerpc

Large physical address support

configname: CONFIG_PHYS_64BIT

Linux Kernel Configuration
└─> Processor support
└─> Large physical address support

This option enables kernel support for larger than 32-bit physical
addresses. This feature may not be available on all cores.

If you have more than 3.5GB of RAM or so, you also need to enable
SWIOTLB under Kernel Options for this to work. The actual number
is platform-dependent.

If in doubt, say N here.