kernel: enable SERIAL_8250_16550A_VARIANTS

Kernel 5.6 introduced a new config symbol SERIAL_8250_16550A_VARIANTS.
In kernel 5.8, this symbol was changed to default to on on !x86, as some
embedded devices still use 16650A variants. Let's play safe here and
enable this symbol in the generic config, to avoid others from running
into this problem and having to spend several hours trying to bisect
this problem. While we could disable the symbol in the x86 target
configs, a 20ms boot time reduction really isn't worth the time wasted
on bisecting this issue.

Matt discovered this problem while working on adding support for the
WatchGuard Firebox M200 to the qoriq target, where it caused some
characters to be missing on the console output.

Reported-by: Matt Fawcett <mattytap@icloud.com>
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Reviewed-by: Rui Salvaterra <rsalvaterra@gmail.com>

target/linux/generic/config-5.10

@@ -5314,7 +5314,7 @@ CONFIG_SELECT_MEMORY_MODEL=y
 # CONFIG_SENSORS_XGENE is not set
 # CONFIG_SENSORS_ZL6100 is not set
 CONFIG_SERIAL_8250=y
-# CONFIG_SERIAL_8250_16550A_VARIANTS is not set
+CONFIG_SERIAL_8250_16550A_VARIANTS=y
 # CONFIG_SERIAL_8250_ACCENT is not set
 # CONFIG_SERIAL_8250_ASPEED_VUART is not set
 # CONFIG_SERIAL_8250_BOCA is not set