Define a dummy version of the AArch64 OSLAR_EL1 system register
which just ignores writes. Linux will always write to this (it
is the OS lock used for debugging), but we don't support debug.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>