MIPS: Use memblock_phys_alloc() for exception vector

Allocate the exception vector using memblock_phys_alloc() which gives us
a physical address, rather than the previous convoluted setup which
obtained a virtual address using memblock_alloc(), converted it to a
physical address & then back to a virtual address.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Cc: linux-mips@vger.kernel.org