MIPS: Sync icache for whole exception vector

Rather than performing cache flushing for a fixed 0x400 bytes, use the
actual size of the vector in order to ensure we cover all emitted code
on systems that make use of vectored interrupts.

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