ARM OMAP2+ GPMC: fix programming/showing reserved timing parameters

GPMC_CONFIG1_i parameters CLKACTIVATIONTIME and WAITMONITORINGTIME
have reserved values.
Raise an error if calculated timings try to program reserved values.

GPMC_CONFIG1_i ATTACHEDDEVICEPAGELENGTH and DEVICESIZE were already checked
when parsing the DT.

Explicitly comment invalid values on gpmc_cs_show_timings for
-CLKACTIVATIONTIME
-WAITMONITORINGTIME
-DEVICESIZE
-ATTACHEDDEVICEPAGELENGTH

Signed-off-by: Robert ABEL <rabel@cit-ec.uni-bielefeld.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>