davinci: da8xx/omap-l1: fix build error when CONFIG_DAVINCI_MUX is undefined

The da8xx/omap-l1 boards refuse to build when CONFIG_DAVINCI_MUX is undefined
because arch/arm/mach-davinci/mux.c:da8xx_pinmux_setup() is not defined.

This patch fixes this issue. This is build tested with davinci_all_defconfig
and da8xx_omapl_defconfig and boot tested on DA830 EVM.

Reported-by: Shanmuga Sundaram Mahendran <shanmuga@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>