PM: EM: Implement em_nl_get_pds_doit()

PM: EM: Implement em_nl_get_pds_doit()

When a userspace requests EM_CMD_GET_PDS, the kernel responds with
information on all performance domains. The message format of the
response is as follows:

EM_A_PDS_PD (NLA_NESTED)*
    EM_A_PD_PD_ID (NLA_U32)
    EM_A_PD_FLAGS (NLA_U64)
    EM_A_PD_CPUS (NLA_STRING)

where EM_A_PDS_PD can be repeated as many times as there are performance
domains, and EM_A_PD_CPUS is a hexadecimal string representing a CPU
bitmask.

Signed-off-by: Changwoo Min <changwoo@igalia.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/20251020220914.320832-7-changwoo@igalia.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>