fix(imx8m): fix dram retention fsp_table access

The fsp_table access by [i-1] can cause invalid memory access in case of
i=0. This can be the case if no fsp_table is available. Fix this by
adding the idx variable which tracks the correct index.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: If2285517eb9fe837f3ad54360307a77a658bf62c