ACPI: SPCR: Make SPCR available to x86

SPCR is currently only enabled or ARM64 and x86 can use SPCR to setup
an early console.

General fixes include updating Documentation & Kconfig (for x86),
updating comments, and changing parse_spcr() to acpi_parse_spcr(),
and earlycon_init_is_deferred to earlycon_acpi_spcr_enable to be
more descriptive.

On x86, many systems have a valid SPCR table but the table version is
not 2 so the table version check must be a warning.

On ARM64 when the kernel parameter earlycon is used both the early console
and console are enabled.  On x86, only the earlycon should be enabled by
by default.  Modify acpi_parse_spcr() to allow options for initializing
the early console and console separately.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mark Salter <msalter@redhat.com>
Tested-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>