drivers/spi: Add support for GRLIB SPIMCTRL

This adds support for the GRLIB SPIMCTRL SPI controller used in LEON and
NOEL-V systems. SPIMCTRL can operate in two different modes: In the
default mode it allows memory-mapped read access to the flash data. When
set in the user mode, it can be used to generate SPI bus transactions.

Signed-off-by: Martin Åberg <martin.aberg@gaisler.com>