imx6: move all standard includes to mx6_common.h

The linux/sizes.h, asm/arch/imx-regs.h, asm/imx-common/gpio.h,
config_cmd_default.h includes are used fairly universally across imx6 boards
so include them in mx6_common.h by default.

We define CONFIG_SYS_NO_FLASH before config_cmd_default.h  so that we
don't have to undef CONFIG_CMD_FLASH / CONFIG_CMD_IMLS everywhere.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>