MIPS: Make the default for PHYSICAL_START always 64-bit

Make the default for PHYSICAL_START always 64-bit, ensuring that a
correct sign-extended value is used if a 32-bit image is loaded by a
64-bit system, and matching how the load address is set in platform
Makefile fragments (arch/mips/*/Platform) in the absence of the
PHYSICAL_START configuration option.

Of course PHYSICAL_START itself is a misnomer as the load address is
virtual rather than physical (or otherwise sign-extension would not
apply).

Fixes: 7aa1c8f4

 ("MIPS: kdump: Add support")
Signed-off-by: Maciej W. Rozycki <macro@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Maxim Uvarov <muvarov@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/18939/
Signed-off-by: James Hogan <jhogan@kernel.org>