dmaengine: pl08x: be fair when re-assigning physical channel

Current way we find a waiting virtual channel for the next transfer
at the time one physical channel becomes free is not really fair.

More in details, in case there is more than one channel waiting at a time,
by just going through the arrays of memcpy and slave channels and stopping
as soon as state match waiting state, channels with high indexes can be
penalized.

Whenever dma engine is substantially overloaded so that we constantly
get several channels waiting, channels with highest indexes might not
be served for a substantial time which in the worse case, might hang
task that wait for dma transfer to complete.

This patch makes physical channel re-assignment more fair by storing
time in jiffies when a channel is put in waiting state. Whenever a
physical channel has to be re-assigned, this time is used to select
channel that is waiting for the longest time.

Signed-off-by: Jean-Nicolas Graux <jean-nicolas.graux@st.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Nicolas Guion <nicolas.guion@st.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>