sunxi: Fix CONFIG_UART0_PORT_F build and add it to menuconfig

The CONFIG_UART0_PORT_F option has been supported since
    http://git.denx.de/?p=u-boot.git;a=commit;h=ff2b47f6a9cc1025

This option is primarily useful only for low level u-boot debugging
on tablets, where normal UART0 is difficult to access and requires
device disassembly and/or soldering.

This patch now allows it to be selected from menuconfig. A dependency on
SPL_FEL is added because u-boot does not support booting from NAND yet
and also booting from SD card is impossible when a MicroSD breakout board
is plugged into the SD slot.

Additionally a compilation problem is fixed:

common/spl/built-in.o: In function `spl_mmc_load_image':
/tmp/u-boot-sunxi/common/spl/spl_mmc.c:94: undefined reference to `mmc_initialize'
/tmp/u-boot-sunxi/common/spl/spl_mmc.c:96: undefined reference to `find_mmc_device'
/tmp/u-boot-sunxi/common/spl/spl_mmc.c:104: undefined reference to `mmc_init'
scripts/Makefile.spl:206: recipe for target 'spl/u-boot-spl' failed

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>