ARM: at91: make ATAGS support conditional on CONFIG_ATAGS

We have the global CONFIG_ATAGS symbol that is used to determine
whether ATAGS based boot is possible or not. Since we are
splitting out the legacy boards from the main Kconfig on at91,
we can use that symbol to easily hide the old options.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>