MIPS: R2: Fix problem with code that incorrectly modifies ebase.

Commit 566f74f6 had a change that
incorrectly modified ebase. This backs out the lines that modified
ebase.
In addition, the ebase exception vector is now allocated with correct
alignment and the ebase register updated according to the architecture
specification.

Based on original patch by David VomLehn <dvomlehn@cisco.com>.

Signed-off-by: David VomLehn <dvomlehn@cisco.com>
Signed-off-by: Chris Dearman <chris@mips.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>