lib: utils: fdt_pmu: Make the fdt_pmu_evt_select table global variable

To allow platform override pmu_init() filling the translation table
fdt_pmu_evt_select[] when PMU node doesn't provide such information,
we need to share the table and its entry counter with other .c file.

We also define the structures of PMU property in fdt_helper.h, so we
can initialize the mappings in arrays.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>