In contrast to other architectures, R.Pi debug UART config was placed
under the "System Type" menu, not under the "Debugging -> low-level
debugging port". This made this setting easy to miss when enabling low
level debug mesages.

While at it use the existing base address defines rather than defining
them again in Kconfig.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>