device-tree: nexus7: Add bq27541 battery interface to dts

Add support for battery level reading on the Nexus7 by
enabling the bq27541 driver in the nexus7 dts

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Arnd Bergmann <arnd.bergmann@linaro.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Andy Gross <agross@codeaurora.org>
Cc: Vinay Simha BN <simhavcs@gmail.com>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>
Cc: Stephen Boyd <stephen.boyd@linaro.org>
Cc: linux-arm-msm@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: John Stultz <john.stultz@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>