ARM: add ATAGS dependencies to non-DT platforms

ARM: add ATAGS dependencies to non-DT platforms

There are a total of eight platforms that only suppor ATAGS based boot
with board files but no devicetree booting.

For dove, the DT support is part of the mvebu platform, which shares
driver but no code in arch/arm.

Most of these will never get converted to DT, and the majority of the
board files appear to be entirely unused already. There are still known
users on a few machines, and there may be interest in converting some
omap1, ep93xx or footbridge machines over in the future.

For the moment, just add a Kconfig dependency to hide these platforms
completely when CONFIG_ATAGS is disabled, and reorder the priority
of the options: Rather than offering to turn ATAGS off for platforms
that have DT support, make it a top-level setting that determines
which platforms are visible.

The s3c24xx platform supports one machine with DT support, but it
cannot be built without also including ATAGS support, and the
entire platform is scheduled for removal, so leaving the entire
platform behind a dependency seems good enough.

All defconfig files should keep working, as the option remains default
enabled.

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