dts: bindings: semtech,sx1276: Remove redundant document separator

Not needed. Bindings are never concatenated together.

Makes the https://github.com/zephyrproject-rtos/ci-tools/pull/123

 check
clean, though it only looks at changed files in PRs.

Clean up the header too.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>