ARM: dts: da850: specify the maximum pixel clock rate for tilcdc

At maximum CPU frequency of 300 MHz the maximum pixel clock frequency
is 37.5 MHz[1]. We must filter out any mode for which the calculated
pixel clock rate would exceed this value.

Specify the max-pixelclock property for the display node for
da850-lcdk.

[1] http://www.ti.com/lit/ds/symlink/am1808.pdf
(SPRS653E, revised March 2014, table 6-110)

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
[nsekhar@ti.com: commit message update]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>