dts: adsp: ace15: remove lp clock

LP/HP RING OSC clocks were replaced by the ACE IPLL clock.

If needed IPLL can be configured to work as low power clock. But right
now ACE uses only WOVCRO and IPLL (configured to work as HP RING OSC
clock).

Signed-off-by: Tomasz Leman <tomasz.m.leman@intel.com>