rpmsg: char: Switch to SPDX license identifier

Use the appropriate SPDX license identifier in the rpmsg char driver
source file and drop the previous boilerplate license text. The uapi
header file already had the SPDX license identifier added as part of
a mass update but the license text removal was deferred for later,
and this patch drops the same.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>