tests: drivers: uart: boards: Disable Userspace

This commit disables CONFIG_TEST_USERSPACE in
intel_rpl_s_crb.conf of uart_async_api
testcase.Enabling this config introduces
restrictions that interfere with cacheable
regions by blocking access and modifying
cache attributes.By disabling this
configuration, the following issues
are resolved:
-Cacheable region retain their attributes.
-Execution and data transactions work without
restrictions.
-System behavior align with expected
configuration in privileged mode.
-Some code primarily relying on non-cache
region continues to work.

This change is neccessary to ensure
cachebale memory regions function as intended
without interferance from user mode restrictions.

Signed-off-by: S Swetha <s.swetha@intel.com>