dmaengine/amba-pl08x: Add prep_single_byte_llis() routine

Code for creating single byte llis is present at several places. Create a
routine to avoid code redundancy.

Also, we don't need one lli per single byte transfer, we can have single lli to
do all single byte transfer.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>