target/riscv: UPDATE xATP write CSR

target/riscv: UPDATE xATP write CSR

Added xATP_MODE validation for vsatp/hgatp CSRs.
The xATP register is an SXLEN-bit read/write WARL register, so
the legal value must be returned (See riscv-privileged-20211203, SATP/VSATP/HGATP CSRs).

Signed-off-by: Irina Ryapolova <irina.ryapolova@syntacore.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240109145923.37893-2-irina.ryapolova@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>