DA8xx: switch an enum to defines for consistency

There are two main sets of LPSC constants, depending on the processor
family.  The DA8xx constants were given in an enum whereas the non-DA8xx
constants were preprocessor defines. This commit switches the DA8xx
constants to defines for consistency.

Signed-off-by: Laurence Withers <lwithers@guralp.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>