riscv: don't export va_kernel_pa_offset in vmcoreinfo for XIP kernel

The crash utility uses va_kernel_pa_offset to translate virtual addresses.
This is incorrect in the case of XIP kernel, because va_kernel_pa_offset is
not the virtual-physical address offset (yes, the name is misleading; this
variable will be removed for XIP in a following commit).

Stop exporting this variable for XIP kernel. The replacement is to be
determined, note it as a TODO for now.

Signed-off-by: Nam Cao <namcao@linutronix.de>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/8f8760d3f9a11af4ea0acbc247e4f49ff5d317e9.1717789719.git.namcao@linutronix.de


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>