tests/drivers/uart/uart*: Allow on nrf52_bsim

Add the nrf52_bsim to the platform_allow list
for tests which had such a filter.

Note that for this test to pass, you need
to connect the uart in loopback, passing the command
line option `-uart0_loopback` to zephyr.exe

Note this tests do not run right now by default in CI
as the fixture is preventing it.
They can be run with:

twister -T tests/drivers/uart/ -v -p nrf52_bsim \
  --fixture gpio_loopback -- -uart0_loopback

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>