arm64: dts: qcom: msm8998: correct xo clock name

The root parent clock of most msm8998 clock is the "xo" clock.  The DT node
is incorrectly named "xo_board", which prevents Linux from correctly
parsing the clock tree, resulting in most clocks being unparented and
unable to be manipulated.  The end result is that we can't turn on clocks
for peripherals like SD, so init usually fails.

Fixes: 4807c71c (arm64: dts: Add msm8998 SoC and MTP board support)
Signed-off-by: Jeffrey Hugo <jhugo@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>