spi: bcm53xx: driver for SPI controller on Broadcom bcma SoC

Broadcom 53xx ARM SoCs use bcma bus that contains various cores (AKA
devices). If board has a serial flash, it's connected over SPI and the
bcma bus includes a SPI controller. Example log from such a board:
bus0: Found chip with id 53010, rev 0x00 and package 0x02
(...)
bus0: Core 18 found: SPI flash controller (manuf 0x4BF, id 0x50A, rev 0x01, class 0x0)

This patch adds a bcma driver for SPI core, it registers SPI master
controller and "bcm53xxspiflash" SPI device.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>