platform: andes: Add Andes custom PMU support

Before the ratification of Sscofpmf, the Andes PMU extension
was designed to support the sampling and filtering with hardware
performance counters (zihpm), it works with the current SBI PMU
extension and Linux SBI PMU driver.

We implement 1) the PMU device callbacks that update the
corresponding bits on custom CSRs, 2) extentions_init() to detect
the hardware support of Andes PMU and initialize the per-hart
PMU related CSR, and 3) pmu_init() to register PMU device and
populate event mappings.

Also define a andes_pmu_setup() function which is in preparation
for adding default PMU mappings in andes_hpm.h

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>