arm64: Use __pa_symbol for empty_zero_page

If CONFIG_DEBUG_VIRTUAL=y and CONFIG_ARM64_SW_TTBR0_PAN=y:

    virt_to_phys used for non-linear address: ffffff8008cc0000 (empty_zero_page+0x0/0x1000)
    WARNING: CPU: 0 PID: 0 at arch/arm64/mm/physaddr.c:14 __virt_to_phys+0x28/0x60
    ...
    [<ffffff800809abb4>] __virt_to_phys+0x28/0x60
    [<ffffff8008a02600>] setup_arch+0x46c/0x4d4

Fixes: 2077be67 ("arm64: Use __pa_symbol for kernel symbols")
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Will Deacon <will.deacon@arm.com>