dts: nordic: nrf54h20: define hsfll120 instance

HSFLL120 is a clock used by fast peripherals, and it is controlled by
the system controller. From an application perspective, it is a fixed
clock. Note that it has multiple outputs (clk_main @ 320MHz, and
clk_main2|4 which provide the main clock divided by 2 and 4,
respectivelu). Only the main frequency is represented for now.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>