ram: stm32mp1: tuning: add timeout for polling BISTGSR.BDDONE

Avoid to block the tuning procedure on BIST error (not finished
BIST procedure) by adding a 1000us timeout on the polling of
BISTGSR.BDDONE executed to detect the end of BIST.

The normal duration of the BIST test is around 5us.

This patch also cleanup comments.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Patrice Chotard <patrice.chotard@st.com>