da850-evm: KEYBOARD_GPIO_POLLED Kconfig conditional

Use the mach-davinci/Kconfig to enable gpio-keys-polled as default when
da850-evm machine is enabled.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
CC: Kevin Hilman <khilman@deeprootsystems.com>
CC: "Nori, Sekhar" <nsekhar@ti.com>
CC: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>