selftests: pstore: Adding config fragment CONFIG_PSTORE_RAM=m

pstore_tests and pstore_post_reboot_tests need CONFIG_PSTORE_RAM=m

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>