ARM: debug-ll: fix UART configuration with ARCH_KEYSTONE

We may have multiple platforms enabled and also DEBUG_LL
configured for one of them. However if we enable ARCH_KEYSTONE,
we default to using 32-bit UART access independent of which
platform we are actually using, which can be confusing.

This changes the logic so the 32-bit default gets only
used by default if we actually configure the keystone
UART, as opposed to picking some other 8250 setting on
a kernel that has keystone support enabled.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>