tests: gpio_basic_api: change GPIOs for mec15xxevb_assy6853

GPIO 050/051 are being used for tachometer sensor as
CONFIG_TACH_XEC is enabled by default. So for the gpio_basic_api
test, another set of GPIOs are needed. GPIO 156/157 are chosen
as they are (more or less) dedicated for two LEDs on board and
not being used for other functions (pinmux-wise).

Fixes #25272

Signed-off-by: Daniel Leung <daniel.leung@intel.com>