xen/arm32: entry: Document the purpose of r11 in the traps handler

xen/arm32: entry: Document the purpose of r11 in the traps handler

It took me a bit of time to understand why __DEFINE_TRAP_ENTRY is
storing the original stack pointer in r11. It is working in pair with
return_traps_entry where sp will be restored from r11.

This is fine because per the AAPCS r11 must be preserved by the
subroutine. So in return_from_trap, r11 will still contain the original
stack pointer.

Add some documentation in the code to point the 2 sides to each other.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>