rtc: optee: fix memory leak on driver removal

Fix a memory leak in case of driver removal.
Free the shared memory used for arguments exchanges between kernel and
OP-TEE RTC PTA.

Fixes: 81c2f059 ("rtc: optee: add RTC driver for OP-TEE RTC PTA")
Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://lore.kernel.org/r/20250715-upstream-optee-rtc-v1-1-e0fdf8aae545@foss.st.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>