ARM: dts: atlas7: add fixed frequency clocks in car node

This patch adds the fixed clocks of external crystal oscillators.

Signed-off-by: Guo Zeng <Guo.Zeng@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
[sboyd@codeaurora.org: Remove size-cells/address-cells]
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>