SBBR mentions SPCR as a mandatory ACPI table.  So enable it for ARM64

Earlycon should be set up as early as possible.  ACPI boot tables are
mapped in arch/arm64/kernel/acpi.c:acpi_boot_table_init() that
is called from setup_arch() and that's where we parse SPCR.
So it has to be opted-in per-arch.

When ACPI_SPCR_TABLE is defined initialization of DT earlycon is
deferred until the DT/ACPI decision is done.  Initialize DT earlycon
if ACPI is disabled.
Acked-by: NWill Deacon <will.deacon@arm.com>
Acked-by: NHanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: NAleksey Makarov <aleksey.makarov@linaro.org>
Tested-by: NKefeng Wang <wangkefeng.wang@huawei.com>
Tested-by: NChristopher Covington <cov@codeaurora.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>