ARM: no longer make CPU targets visible separately

Now that realview and integrator always select the correct CPU
type themselves based on the core tiles, there is no need to
still have them user-visible in arch/arm/mm/Kconfig. The
ARM925T symbol has been selected by the only user for many
years, so that can be removed along with the realview and
integrator specific ones.

This also solves randconfig build problems on realview.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>