writing_musb_glue_layer.rst: Fix broken URL

The http://processors.wiki.ti.com

 EOL in january 2021

The old documentation is still available through the wayback machine.

Signed-off-by: Richard Genoud <richard.genoud@bootlin.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240618151938.1827064-1-richard.genoud@bootlin.com