net: ti: icss-iep: constify struct regmap_config

`am654_icss_iep_regmap_config` is only assigned to a pointer that passes
the data as read-only.

Add the const modifier to the struct and pointer to move the data to a
read-only section.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: MD Danish Anwar <danishanwar@ti.com>
Link: https://patch.msgid.link/20240703-net-const-regmap-v1-2-ff4aeceda02c@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>