MIPS: Consider value of c0_ebase when computing value of exception base.

It just so happens to be zero on all currently supported systems so this
hasn't bitten yet ...

[Ralf: Original patch from Cavium; handling of set_uncached_handler() and
de-ifdef'ed trap_init() implementation by me.]

Signed-off-by: Tomaso Paoletti <tpaoletti@caviumnetworks.com>
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>