mips: econet: Fix incorrect Kconfig dependencies

mips: econet: Fix incorrect Kconfig dependencies

config ECONET selects SERIAL_OF_PLATFORM and that depends on SERIAL_8250
so we need to select SERIAL_8250 directly.
Also do not enable DEBUG_ZBOOT unless DEBUG_KERNEL is set.

Signed-off-by: Caleb James DeLisle <cjd@cjdns.fr>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505211654.CBdIsoTq-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202505211451.WRjyf3a9-lkp@intel.com/
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>