tty: serial: samsung_tty: Add earlycon support for Apple UARTs

Earlycon support is identical to S3C2410, but Apple SoCs also need
MMIO mapped as nGnRnE. This is handled generically for normal drivers
including the normal UART path here, but earlycon uses fixmap and
runs before that scaffolding is ready.

Since this is the only case where we need this fix, it makes more
sense to do it here in the UART driver instead of introducing a
whole fdt nonposted-mmio resolver just for earlycon/fixmap.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210304213902.83903-26-marcan@marcan.st
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>