ARM: dts: ux500/golden: Set display max brightness

A too high brightness by default (default is max) makes the
screen go blank. Set this to 15 as in the Vendor tree.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20201214223413.253893-1-linus.walleij@linaro.org

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>