Fix existence check for MMIO-mapped 16550 UARTs

Fix existence check for MMIO-mapped 16550 UARTs

Changeset 982e6fce0e47 added an existence test for UARTs.
Unfortunately, the existence test happens before MMIO UARTs are
ioremapped, therefore it may not be probing where it thinks it's
probing.  Rather than moving more code around, I think it's probably
safe to assume the arch code knows what it's doing if it passes in an
MMIO UART.

Signed-off-by: Alex Williamson <alex.williamson@hp.com>