bootm: Disable interrupts only when loading

With the move of the interrupt code to earlier in the sequence, we
exposed a problem where the interrupts are disabled at each bootm
stage. This is not correct - it should be done only once. Let's disable
interrupts in the LOAD stage. Put the code in a function for clarity.

Also, bootz lost its interrupt code altogether, so reinstate it.

Signed-off-by: Simon Glass <sjg@chromium.org>