firmware: arm_ffa: Simplify the computation of transmit and fragment length

The computation of endpoint memory access descriptor's composite memory
region descriptor offset is using COMPOSITE_CONSTITUENTS_OFFSET which is
unnecessary complicated. Composite memory region descriptor always follow
the endpoint memory access descriptor array and hence it is computed
accordingly. COMPOSITE_CONSTITUENTS_OFFSET is useless and wrong for any
input other than endpoint memory access descriptor count.

Let us drop the usage of COMPOSITE_CONSTITUENTS_OFFSET to simplify the
computation of total transmit and fragment length in the memory
transactions.

Link: https://lore.kernel.org/r/20231005-ffa_v1-1_notif-v4-13-cddd3237809c@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>