kernel: fix earlycon by keeping __earlycon_table

Somewhere between 4.4 and 4.9 there was a change that made
OF_EARLYCON_DECLARE adding entries to the __earlycon_table instead of
__earlycon_of_table. We need to keep this table as well.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>