ARM: debug-ll: rework ep93xx handling

This makes ep93xx debug-ll handling more consistent with the other
platforms, by adding a separate Kconfig symbol for it that
in turn selects the standard DEBUG_UART_PL01X symbol.

We still have to pick a physical address even if DEBUG_LL is disabled
here, because the EP93xx uncompress output code uses
CONFIG_DEBUG_UART_PHYS. If we ever move to multiplatform support,
this can go away.

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