There is "cpu - possible values:
all (default), percpu (total load divided by online CPU count)" in the documentation of item key system.cpu.load[<cpu>,<mode>] https://www.zabbix.com/documentation/2.0/manual/config/items/itemtypes/zabbix_agent.

But when configuring on Zabbix server via frontend, the error message is:

Incorrect item key "system.cpu.load[all,avg1]" provided for trigger expression on "Template OS Linux".
or
Incorrect item key "system.cpu.load[,avg1]" provided for trigger expression on "Template OS Linux".

percpu works.