[MIPS] Fix linker script to work for non-4K page size.

    
Very much to my surprise Fuxin Zhang reports this is all it takes to get
the kernel to work for page sizes larger than 4kB.  This also paves the
way for support for the R6000 and R8000 which don't support 4kB page size.
    
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>