i2c: acorn: add MODULE_LICENSE tag

As of v4.15, Kbuild warns about missing MODULE_LICENSE tags:

WARNING: modpost: missing MODULE_LICENSE() in drivers/i2c/busses/i2c-acorn.o

This adds a license, author and description tag, matching the
comment at the start of the acorn i2c driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>