ARM: OMAP2+: Add missing LCDC midlemode for am335x

TRM "Table 13-34. SYSCONFIG Register Field Descriptions" lists both
standbymode and idlemode that should be just the sidle and midle
registers where midle is currently unconfigured for lcdc_sysc. As
the dts data has been generated based on lcdc_sysc, we now have an
empty "ti,sysc-midle" property.

And so we currently get a warning for lcdc because of a difference
with dts provided configuration compared to the legacy platform
data. This is because lcdc has SYSC_HAS_MIDLEMODE configured in
the platform data without configuring the modes.

Let's fix the issue by adding the missing midlemode to lcdc_sysc,
and configuring the "ti,sysc-midle" property based on the TRM
values.

Fixes: f711c575

 ("ARM: dts: am335x: Add l4 interconnect hierarchy and ti-sysc data")
Cc: Jyri Sarha <jsarha@ti.com>
Cc: Keerthy <j-keerthy@ti.com>
Cc: Robert Nelson <robertcnelson@gmail.com>
Cc: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>