riscv/mm: Add XIP_FIXUP for phys_ram_base

This manifests as a crash early in boot on VexRiscv.
Signed-off-by: Myrtle Shah <gatecat@ds0.me>
[Palmer: split commit]
Fixes: 6d7f91d9 ("riscv: Get rid of CONFIG_PHYS_RAM_BASE in kernel physical address conversion")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/mm/init.c

@@ -522,6 +522,7 @@ static uintptr_t __init best_map_size(phys_addr_t base, phys_addr_t size)
 }
 
 #ifdef CONFIG_XIP_KERNEL
+#define phys_ram_base  (*(phys_addr_t *)XIP_FIXUP(&phys_ram_base))
 extern char _xiprom[], _exiprom[], __data_loc;
 
 /* called from head.S with MMU off */