Davinci: serial - use ioremap()

This patch implements davinci serial cleanups towards having this code
reusable on tnetv107x.

The change reuses the platform data membase field to hold the remapped space.
By disabling the UPF_IOREMAP flag in the platform data, we prevent
the 8250 driver from repeating the ioremap.

Signed-off-by: Cyril Chemparathy <cyril@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>