OMAP: DSS2: Convert simple/strict_strto* to kstrto*

Convert simple/strict_strto* functions to kstrto* functions. Only simple
cases are converted.

simple_strto* uses are still left to places where it is used to parse
numbers from a list of numbers. These need some other solution than
kstrto*.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>