selftests: memory-hotplug: add required configs

add CONFIG_MEMORY_HOTREMOVE=y in config
without this config, /sys/devices/system/memory/memory*/removable
always return 0, I endup getting an early skip during test

Signed-off-by: Lei Yang <Lei.Yang@windriver.com>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>