ARM: davinci: always select CONFIG_CPU_ARM926T

The select was lost by accident during the multiplatform conversion.
Any davinci-only

arm-linux-gnueabi-ld: arch/arm/mach-davinci/sleep.o: in function `CACHE_FLUSH':
(.text+0x168): undefined reference to `arm926_flush_kern_cache_all'

Fixes: f962396c

 ("ARM: davinci: support multiplatform build for ARM v5")
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20240108110055.1531153-1-arnd@kernel.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>