tty: serial: bcm63xx: use more precise Kconfig symbol

tty: serial: bcm63xx: use more precise Kconfig symbol

Patches lowering SERIAL_BCM63XX dependencies led to a discussion and
documentation change regarding "depends" usage. Adjust Kconfig entry to
match current guidelines. Make this symbol available for relevant
architectures only.

Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Ref: f35a07f92616 ("tty: serial: bcm63xx: lower driver dependencies")
Ref: 18084e435ff6 ("Documentation/kbuild: Document platform dependency practises")
Link: https://lore.kernel.org/r/20220311093233.10012-1-zajec5@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>