slimbus: Specify number of channels for the group in group handle

Use group handle instead of next element in channel's data structure
to identify how many channels were specified by a client in the group.
That way clients can specify different number of channels for shared
channels, and work with their own client-handle. Note that sequence of
channel still matters (so 2 clients sharing a channel should not
specify channels in different order, otherwise scheduling of the
channels will not be in the correct order)

CRs-Fixed: 415109
Change-Id: Idd636a500ee6449b16648a234b9e76fad21f2fcc
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>