tests: drivers: uart: async_api: update wba55cg clock frequency

With the current configuration, we encounter a user setting error
during the test with the log:
"Wrong number of bytes received, got: 2, expected: 3."

Workaround:
Increase the clock frequency to enable faster data transmission
and avoid user setting errors.

Signed-off-by: Fabrice DJIATSA <fabrice.djiatsa-ext@st.com>