drivers: memc: siwx91x: Do not override clock configuration

In the original HAL, sl_si91x_psram_init() and sl_si91x_psram_uninit() were
also in charge of configuring the pinctrl and the clocks. A workaround
have been introduced to avoid change in pinctrl but they still changed the
clock configuration.

We definitely need to expose the clock configuration to Zephyr users. The
HAL has been patched to split the sl_si91x_psram_*init() function in
smaller pieces. So it is possible to configure the devic without changing
the clock or the pinctrl. Let's use these new functions.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>