tests/qtest: Fix STM32L4x5 GPIO test on 32-bit

tests/qtest: Fix STM32L4x5 GPIO test on 32-bit

The test mangles the GPIO address and the pin number in the
qtest_add_data_func data parameter. Doing so, it assumes that the host
pointer size is always 64-bit, which breaks on 32-bit :

../tests/qtest/stm32l4x5_gpio-test.c: In function ‘test_gpio_output_mode’:
../tests/qtest/stm32l4x5_gpio-test.c:272:25: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
  272 |     unsigned int pin = ((uint64_t)data) & 0xF;
      |                         ^
../tests/qtest/stm32l4x5_gpio-test.c:273:22: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
  273 |     uint32_t gpio = ((uint64_t)data) >> 32;
      |                      ^

To fix, improve the mangling of the GPIO address and pin number fields
by using GPIO_SIZE so that the resulting value fits in a 32-bit pointer.
While at it, include some helpers to hide the details.

Cc: Arnaud Minier <arnaud.minier@telecom-paris.fr>
Cc: Inès Varhol <ines.varhol@telecom-paris.fr>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Message-id: 20240329092747.298259-1-clg@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>