dts: weact_stm32h5_core: Define clk_hsi48 to fix USB support

The USB CDC driver relies on the HSI48 internal oscillator as the
default clock source for USB functionality. In the weact_stm32h5_core
device tree, the clk_hsi48 node was missing, resulting in USB
initialization failure.

Signed-off-by: Filip Stojanovic <filipembedded@gmail.com>