modules: tf-m: add TFM_BL2 dependency on TFM_MCUBOOT_IMAGE_NUMBER

TFM_MCUBOOT_IMAGE_NUMBER Kconfig option should have
a dependency on TFM_BL2, since it is not applicable
otherwise.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>