spi: bcm53xx: (re)license code to the GPL v2

My intention was to release this code under GPL v2 license. For some
reason my initial commit 0fc6a323 ("spi: bcm53xx: driver for SPI
controller on Broadcom bcma SoC") totally missed licensing info.
MODULE_LICENSE was later added by Axel specifying "GNU Public License
v2 or later".

This patch clarifies situation by adding a proper header (with Copyright
line) and adjusting MODULE_LICENSE. It should be acked by every driver
contributor.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Jingoo Han <jingoohan1@gmail.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Acked-by: Joe Perches <joe@perches.com>
Acked-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Vaishali Thakkar <vaishali.thakkar@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>