bcm2835/spi: Support SPI_FLAG_KEEP_CS

Summary:
3c08673438 brought in SPI_FLAG_KEEP_CS to keep the SPI chip select held
post-transfer completion. Add this support to bcm2835 SPI for SPI
devices that need it. As part of this, the owner thread needed carried
through so that no other thread can take over the SPI bus until the
owner releases the chip select.

Reviewed by: manu
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D42599