ARM: aspeed: ast2500 is ARMv6K

Linux supports both the original ARMv6 level (early ARM1136) and ARMv6K
(later ARM1136, ARM1176 and ARM11mpcore).

ast2500 falls into the second categoy, being based on arm1176jzf-s.
This is enabled by default when using ARCH_MULTI_V6, so we should
not 'select CPU_V6'.

Removing this will lead to more efficient use of atomic instructions.

Fixes: 8c2ed9bc

 ("arm: Add Aspeed machine")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Joel Stanley <joel@jms.id.au>