imx8qxp_mek: Fix the console command line string

Currently the expansion of the console variable leads to
the following kernel command line:

console=ttyLP0,${baudrate} earlycon root=/dev/mmcblk1p2 rootwait rw

, which causes the console to not show characters after the LPUART driver
is probed as the 'baudrate' variable is not properly translated.

Fix it by splitting the console variable in two parts: one for the
ttyLP0 part and the other one for the baudrate, which matches the way
it is done on other i.MX targets.

Tested by successfully booting a mainline kernel on a i.MX8QXP MEK
board.

Signed-off-by: Fabio Estevam <festevam@gmail.com>