* SAMV7: SPI: SPI-Freq. 40MHz; VARSELECT; hw-features

  This change adds the following improvements:
  - Increase the allowed SPI-Frequency from 20 to 40 MHz.
  - Correct and rename the "VARSELECT" option This option was included in the code as "CONFIG_SPI_VARSELECT" but nowhere  defined in a Kconfig file. The patch renames it to "CONFIG_SAMV7_SPI_VARSELECT" and corrects the implementation according the datasheet of Atmel. In short, this option switches the processor from "fixed peripheral selection" (single device) to "variable peripheral selection" (multiple devices on the bus).
  - Add a new Function to the interface to control the timing and delays of the chip according the ChipSelect lines. This function can control the delay between the assertion of the ChipSelect and the first bit, between the last bit and the de-assertion of the ChipSelect and between two ChipSelects. This is needed to tune the transfer according the specification of the connected devices.
  - Add three "hw-features" for the SAMV7, which controls the behavior of the ChipSelect:
    1. force CS inactive after transfer: this forces a (short) de-assertion of the CS after a transfer, even if more data is
      available in time
    2. force CS active after transfer: this forces the CS to stay active after a transfer, even if the chip runs out of data. Btw.: this is a prerequisit to make the LASTXFER bit working at all.
    - escape LASTXFER: this suppresses the LASTXFER bit at the end of the  next transfer. The "escape"-Flag is reset automatically.