ARM: Add basic support for Airoha EN7523 SoC

ARM: Add basic support for Airoha EN7523 SoC

EN7523 is an armv8 based silicon used inside broadband access type devices
such as xPON and xDSL. It shares various silicon blocks with MediaTek
silicon such as the MT7622.

Add basic support for Airoha EN7523, enough for booting to console.

The UART is basically 8250-compatible, except for the clock selection.
A clock-frequency value is synthesized to get this to run at 115200 bps.

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Bert Vermeulen <bert@biot.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Link: https://lore.kernel.org/r/20220130145116.88406-4-nbd@nbd.name
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>