dt-bindings: cpufreq: add core count to freq-domain

Add an additional cell to represent the number of expected
core count for the cpu, the core count is used to determine
whether or not a particular performance level is a boost
frequency.

Change-Id: I9745e5d59221def9b9f291acd14a341a20978352
Signed-off-by: David Dai <daidavid1@codeaurora.org>