mtd: Drop unused fsl_upm driver

This is not used since this commit:

   8d1e3cb1 powerpc: mpc83xx: remove MPC8360ERDK, EMPC8360EMDS support

Drop the driver and Kconfig option.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-By: Michael Trimarchi <michael@amarulasolutions.com>