ARM: debug: remove references in DEBUG_UART_8250_SHIFT to removed configs

Commit 67d3928c3df5 ("ARM: omap1: remove unused board files") removes
configs DEBUG_OMAP7XXUART{1,2,3}.

The config DEBUG_UART_8250_SHIFT still refers to those removed configs.

Remove those obsolete references.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>