ARM: debug-ll: reorganize mvebu debug uart config

As we are moving dove/mv78xx0/orion into multiplatform, the debug-ll
configuration options for these platforms are conflicting with the
multiplatform configuration: enabling one of those platforms sometimes
changes the default addresses to the ones used on one of them, rather
than the one that was selected in Kconfig.

This changes the configuration so we share the physical address
configuration with mach-mvebu.

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