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

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

Commit 566f74f6b2f8b85d5b8d6caaf97e5672cecd3e3e 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>