da850-evm: allow pca953x module build

Change the mach-davinci Kconfig file so that GPIO_PCA953X is default when
MACH_DAVINCI_DA850_EVM is set instead of always selecting. This allows users
to compile pca953x as a module.

Signed-off-by: Ben Gardiner <bengardiner@nanometrics.ca>
CC: Sergei Shtylyov <sshtylyov@mvista.com>
CC: Nori, Sekhar <nsekhar@ti.com>
Reviewed-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>