tty: serial: fsl_lpuart: drop earlycon entry for i.MX8QXP

i.MX8QXP lpuart is compatible with i.MX7ULP, so no need
an extra  entry for i.MX8QXP.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/1576822230-23125-4-git-send-email-peng.fan@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>