ARM: stm32: Don't select DMA unconditionally on STM32MP157C

The patch that enabled these had no useful changelog that explains
why it is done, and it causes a build warning:

WARNING: unmet direct dependencies detected for STM32_DMA
  Depends on [n]: DMADEVICES [=n] && (ARCH_STM32 [=y] || COMPILE_TEST [=y])
  Selected by [y]:
  - MACH_STM32MP157 [=y] && ARCH_STM32 [=y] && ARCH_MULTI_V7 [=y]

Generally, platforms should not select arbitrary drivers, so let's
just revert that change.

Fixes: de6037fa ("ARM: stm32: Select DMA, DMAMUX and MDMA support on STM32MP157C")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>