target/riscv: Remove obsolete pointer masking extension code.

target/riscv: Remove obsolete pointer masking extension code.

Zjpm extension is finally ratified. And it's much simplier compared to the experimental one.
The newer version doesn't allow to specify custom mask or base for pointer masking.
Instead it allows only certain options for masking top bits.

Signed-off-by: Alexey Baturo <baturo.alexey@gmail.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250106102346.1100149-2-baturo.alexey@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>