arm: socfpga: use imply instead of select where applicable

arm: socfpga: use imply instead of select where applicable

Kconfig should only 'select' features that are required for an arch.
Standard features that can be disabled without breaking board support
should use 'imply' instead, to allow users to disable it.

These options are changed for mach-socfpga:
- DM_SPI & DM_SPI_FLASH: only required with QSPI support enabled
- SYS_MMCSD_RAW_MODE_U_BOOT_USE_PARTITION: the boot rom supports a
  partitionless mode also, where SPL is located at address 0
- HW_WATCHDOG: while all mainline board defconfigs use it, U-Boot
  should still work without it.

Signed-off-by: Simon Goldschmidt <sgoldschmidt@de.pepperl-fuchs.com>