platform/fremont*: set AP to RSS mhu channel count

The number of doorbell channels between RSS and AP MHU (CMU0) is 16
where as model defaults it to 1. So increase the number of doorbell
channel count for CMU0 to 16.

Signed-off-by: Joel Goddard <Joel.Goddard@arm.com>
Signed-off-by: Vijayenthiran Subramaniam <vijayenthiran.subramaniam@arm.com>
Change-Id: I220c7ebed503c422b2ab7a25e75f2f402918f54c