Update DmaSpi.h

- removed initialization of SPI0_CTAR0 in begin(), this can instead be set by chip select classes or other code
- added pause(), paused(), releaseSpi() and resume() methods
- pause() requests the DMA SPI driver to finish a running transfer and to not start any pending transfers
- paused() return true when no transfer is in progress an a pause was requested
- releaseSpi() clears the SPI's request flags for the DMA controller
- resume() reconfigures the SPI for DMA operation and starts a pending transfer.