tests: drivers: uart_async_api: add tx buffer in nocache memory

Since nucleo_f746zg has NOCACHE_MEM defined
(related to test cases drivers.uart.async_api.nocache_mem
and drivers.uart.async_api.nocache_mem_dt.nucleo_f746zg),
the TX buffer should be placed in a non-cacheable memory region
for the uart_async_var_buf_length testsuite to pass.

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