riscv: thead: Add th.sxstatus CSR emulation

The th.sxstatus CSR can be used to identify available custom extension
on T-Head CPUs. The CSR is documented here:
https://github.com/T-head-Semi/thead-

riscv: thead: Add th.sxstatus CSR emulation

The th.sxstatus CSR can be used to identify available custom extension
on T-Head CPUs. The CSR is documented here:
https://github.com/T-head-Semi/thead-extension-spec/blob/master/xtheadsxstatus.adoc

An important property of this patch is, that the th.sxstatus MAEE field
is not set (indicating that XTheadMae is not available).
XTheadMae is a memory attribute extension (similar to Svpbmt) which is
implemented in many T-Head CPUs (C906, C910, etc.) and utilizes bits
in PTEs that are marked as reserved. QEMU maintainers prefer to not
implement XTheadMae, so we need give kernels a mechanism to identify
if XTheadMae is available in a system or not. And this patch introduces
this mechanism in QEMU in a way that's compatible with real HW
(i.e., probing the th.sxstatus.MAEE bit).

Further context can be found on the list:
https://lists.gnu.org/archive/html/qemu-devel/2024-02/msg00775.html

Reviewed-by: LIU Zhiwei <zhiwe_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
Message-ID: <20240429073656.2486732-1-christoph.muellner@vrull.eu>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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