arm: mvebu: Don't use CONFIG_TIMER on ARM64

The 64-bit mvebu SoCs don't have a suitable timer driver so add a !ARM64
condition to the select.

Fixes: 7b530bb1 ("arm: mvebu: Use CONFIG_TIMER on all MVEBU & KIRKWOOD platforms")
Signed-off-by: Chris Packham <judge.packham@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>