target-arm: Implement AArch64 OSLAR/OSLSR_EL1 sysregs

Added oslar_write function to OSLAR_EL1 sysreg, using a status variable
in ARMCPUState.cp15 struct (oslsr_el1). This variable is also linked
to the newly added read-only OSLSR_EL1 register.

Linux reads from this register during its suspend/resume procedure.

Signed-off-by: Davorin Mista <davorin.mista@aggios.com>
[PMM: folded a long line and tweaked a comment]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>