forked from Mirrors/opensbi

The spec states that a6, a7, flags and sepc are writable but the implementation was not allowing that. Add support for these 4 writable attributes. Signed-off-by: Clément Léger <cleger@rivosinc.com> Reviewed-by: Samuel Holland <samuel.holland@sifive.com> Reviewed-by: Anup Patel <anup@brainfault.org>