mfd: Remove dm355evm_msp driver

The DaVinci DM355EVM platform is gone after the removal of all
unused board files, so the MTD device along with its sub-devices
can be removed as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221019152947.3857217-7-arnd@kernel.org