db-mv784mp-gp: Rename CONFIG_DB_784MP_GP to CONFIG_TARGET_DB_MV784MP_GP

The value CONFIG_DB_784MP_GP is only used in the DDR code to refer to
CONFIG_TARGET_DB_MV784MP_GP so just use that second value directly.

Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>