ARM: dts: Fix exynos5250-snow's search key to be L_META

The device tree sent upstream for exynos5250-snow encoded the search
key as CAPSLK.  However in all ChromeOS kernels it is L_META.  One can
certainly have long debates about which it ought to be, but I'm
proposing setting it to L_META because:
* That's how _all_ ChromeOS kernels do it and will do it.
* There is no L_META key on the board, so it's nice to have.
* For those people who really want it to be caps lock, they can use
  xmodmap or somesuch.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>