ARM: mach-davinci: make DAVINCI_UART_INFO into a relative offset

This is the first step to remove PLAT_PHYS_OFFSET usage from the debug
UART code.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>