platform: generic: allwinner: add support for c9xx pmu

With the T-HEAD C9XX cores being designed before or during ratification
of the SSCOFPMF extension, they implement a PMU extension that behaves
very similar but not equal to it by providing overflow interrupts though
in a slightly different registers format.

The sun20i-d1 is using this core. So implement the necessary overrides
to allow its pmu to be used via the standard sbi-pmu extension.

For now it's also the only soc using this core, so keep the additional
code in the d1-space for now.

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>