ARM: dts: keystone: k2g-evm: Move audio oscillator assigned clock to mcasp

Setting assigned clock with a dependency on itself for k2g_clks
creates a circular dependency. Instead, Lets model the audio clock
as a assigned-clock configuration for mcasp and set it up as the
first clock assigned-clock to be satisfied. Following the standard
convention, we use null entries to indicate entries that doesn't need
to be programmed.

Reviewed-by: Tero Kristo <kristo@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>