flex_spi: Support for FlexSPI Flash controller.

NXP FlexSPI is a complex SPI controller which provides
full offload for accessing NOR Flash.
Create a Flash driver which attaches to existing FreeBSD
infrastructure and exports generic READ and WRITE disk commands.
The Flash has to be identified first to configure controller
internals. For now, only one NOR Flash chip is supported.
Future commits shall either increase number of known chips
or implement SFDP mechanism which can be used by other Flash
drivers.

Sponsored by: Alstom
Obtained from: Semihalf
Differential revision: https://reviews.freebsd.org/D33117