tests: gpio-mockup: store the number of chips in gpio_mockup_probe()

tests: gpio-mockup: store the number of chips in gpio_mockup_probe()

We're currently not storing the number of chips in the gpio-mockup
context structure after probing the module. The value is left
initialized to 0. This leads to both a memory leak and to chip ordering
errors when running the tests at high CPU loads. This patch fixes both
issues.

Fixes: e43e46d4f79c ("tests: mockup: add a library for controlling the gpio-mockup module")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>