ARM: dts: msm: Fix msm8x26w GPU DTSI file naming

The GPU device tree file used for msm8x26w targets was
mistakenly given the extension 'dts' rather than 'dtsi', as
would be appropriate for include files.

Change-Id: I2369a4f3725c23ddb3208a15067493fcedcba34c
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>