target: riscv: Add Svrsw60t59b extension support

target: riscv: Add Svrsw60t59b extension support

The Svrsw60t59b extension allows to free the PTE reserved bits 60 and 59
for software to use.

Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Nutty Liu<liujingqi@lanxincomputing.com>
Message-ID: <20250702-dev-alex-svrsw60b59b_v2-v2-1-504ddf0f8530@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>