media: rc keymaps: add SPDX identifiers to the code I wrote

As we're now using SPDX identifiers, on the several
RC keymap files I wrote, add the proper SPDX, identifying
the license I meant.

As we're now using the short license, it doesn't make sense to
keep the original license text.

Also, fix MODULE_LICENSE to identify GPL v2, as this is the
minimal license requirement for those modles.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>