FRV: Make NOMMU-mode work with base addresses other than 0xC0000000 [try #2]

Make NOMMU-mode work with base addresses other than 0xC0000000 by:

 (1) Giving the code that sets up the protection registers the right address
     in __sdram_base.  Rather than being hard coded to 0xC0000000, the value
     of __page_offset is obtained from the linker script.

 (2) Eliminate the check in __switch_to() that verifies the current thread
     info is in the 0xCxxxxxxx region.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>