DaVinci EMAC: declare function for all DA8xx CPUs

The function davinci_emac_mii_mode_sel() is defined in
board/davinci/common/misc.c for any DA8xx CPU which has
CONFIG_DRIVER_TI_EMAC enabled. However, the prototype was only being
declared in <include/asm/arch/davinci_misc.h> for the DA850 EVM board.
This patch declares it for all DA8xx CPUs where CONFIG_DRIVER_TI_EMAC
is enabled.

Signed-off-by: Laurence Withers <lwithers@guralp.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>