serial: arm_dcc: Use CONFIG_ARM64 not CONFIG_CPU_ARMV8

The only place we use CONFIG_CPU_ARMV8 was in the arm_dcc serial driver.
Switch this to CONFIG_ARM64 today, and if in the future we need finer
granularity tuning here, a new CONFIG_SERIAL option needs to be
introduced.

Signed-off-by: Tom Rini <trini@konsulko.com>