lib: utils: fdt_fixup: Allow preserving PMU properties

Add a Kconfig option to control PMU fixup, so the next
stage software can dump the PMU node including event
mapping information for debugging purposes.

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