drivers: tty: serial: IMX_EARLYCON: fix Kconfig dependency on SERIAL_CORE

drivers: tty: serial: IMX_EARLYCON: fix Kconfig dependency on SERIAL_CORE

When SERIAL_IMX_EARLYCON is enabled, and SERIAL_CORE is disabled,
Kbuild gives the following warning:

WARNING: unmet direct dependencies detected for SERIAL_EARLYCON
  Depends on [n]: TTY [=y] && HAS_IOMEM [=y] && SERIAL_CORE [=n]
  Selected by [y]:
  - SERIAL_IMX_EARLYCON [=y] && TTY [=y] && HAS_IOMEM [=y] && (ARCH_MXC [=y] || COMPILE_TEST [=n]) && OF [=y]

This is because SERIAL_IMX_EARLYCON selects SERIAL_EARLYCON,
without selecting SERIAL_CORE, despite that config option depending
on SERIAL_CORE.

Signed-off-by: Julian Braha <julianbraha@gmail.com>
Link: https://lore.kernel.org/r/20210225044248.4745-1-julianbraha@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>