MPC85xx: Change traps.c to not reference non-addressable memory

Currently, END_OF_RAM is used by the trap code to determine if
we should attempt to access the stack pointer or not. However,
on systems with a lot of RAM, only a subset of the RAM is
guaranteed to be mapped in and accessible.  Change END_OF_RAM
to use get_effective_memsize() instead of using the raw ram
size out of the bd.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>