lib: sbi: pmu: Remove unnecessary probe function

The absence of a probe implementation means that the extension is
always available. Remove the implementation for the PMU extension,
which does no checking, and indeed even has a comment saying it's
always available.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>