hwspinlock/sirf: Switch to SPDX license identifier

Use the appropriate SPDX license identifier in the CSR's SIRF hardware
spinlock driver source file and drop the previous boilerplate license text.

Cc: Wei Chen <wei.chen@csr.com>
Cc: Barry Song <baohua@kernel.org>
Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>