platform: andes: Drop andes_pmu_setup()

andes_pmu_setup() [1] was intended to populate event mapping from
hardcoded arrays, however, this increases firmware size and we should
just use PMU DT node [2] instead.

Link: https://lists.infradead.org/pipermail/opensbi/2023-November/006032.html [1]
Link: https://github.com/riscv-software-src/opensbi/blob/v1.4/docs/pmu_support.md#example-3

 [2]
Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>