soc: mec: rename all mec1501x reference to mec15xx

Fix a bunch of references to "mec1501x" to be named "mec15xx" instead,
which is better representative of what is supported and also matches the
soc series name and value.

Fix the clock div option name as well while at it since it apparently
applies to the whole family.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>