ARM: dts: am437x-gp-evm: Add support for buzzer

Add support for onboard gpio buzzer. It works using
the gpio-beeper driver. Pinmux entries for GPIO
controlling the buzzer are also added.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>