lib: sbi: sse: handle missing writable attributes

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>