hw/mcu/nordic: Improve SPIM TX code
hw/mcu/nordic: Improve SPIM TX code

PTR and MAXCNT registers are double buffered so we can setup next chunk
of data just after transmission of current chunk has started. By also
enabling END_START short subsequent transmissions will start immediately
one after another without delay to reconfigure SPIM.

mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core,mlaz/mynewt-core