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: 7aa1c8f ("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>

arch/mips/Kconfig

@@ -2853,8 +2853,7 @@ config CRASH_DUMP
 
 config PHYSICAL_START
 	hex "Physical address where the kernel is loaded"
-	default "0xffffffff84000000" if 64BIT
-	default "0x84000000" if 32BIT
+	default "0xffffffff84000000"
 	depends on CRASH_DUMP
 	help
 	  This gives the CKSEG0 or KSEG0 address where the kernel is loaded.