lib: sbi_pmu: move pmu irq information into pmu itself

Don't spread checking for pmu extensions through the code
but instead introduce a sbi-pmu function that other code can
call to get the correct information about the existence of the
pmu interrupt.

Add a sbi_pmu_device override function to allow overridung this
bit as well if needed.

Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>