arm64: Fix duplicate definition of early_console

When compiling with allmodconfig. early_console is already defined as an
extern global pointer. Need let it point to the object which we intend
to (like arm32 done).

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>