target/riscv: FIX xATP_MODE validation

target/riscv: FIX xATP_MODE validation

The SATP register is an SXLEN-bit read/write WARL register. It means that CSR fields are only defined
for a subset of bit encodings, but allow any value to be written while guaranteeing to return a legal
value whenever read (See riscv-privileged-20211203, SATP CSR).

For example on rv64 we are trying to write to SATP CSR val = 0x1000000000000000 (SATP_MODE = 1 - Reserved for standard use)
and after that we are trying to read SATP_CSR. We read from the SATP CSR value = 0x1000000000000000, which is not a correct
operation (return illegal value).

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: <20240109145923.37893-1-irina.ryapolova@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>