apalis-imx8: fix setting up apalis-uart1

apalis-imx8: fix setting up apalis-uart1

Since RAM initialization can be skipped using SKIP_DDR=1 while
compiling, apalis-uart1 as SCU debug-uart did no longer work. This
commit reworks the solution to stick closer to what NXP does for the
tightly coupled SCU-UART (U=2).

Apalis UART1 can still be enabled with compile option U=6

Relates-to: ELB-4470
Fixes: commit 4aaa1c4 ("mx8qm_apalis: allow debug output on regular console uart")
Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>