xilinx: common: Enabling generic function for DT reselection

xilinx: common: Enabling generic function for DT reselection

U-Boot support board detection at run time and based on it change DT.
This feature is implemented for SOM Kria platforms which contain two
eeproms which contain information about SOM module and CC (Carrier card).
Full U-Boot starts with minimal DT file defined by
CONFIG_DEFAULT_DEVICE_TREE which is available in multi DTB fit image.
It is using default setup of board_name variable initializaed to
DEVICE_TREE which corresponds to CONFIG_DEFAULT_DEVICE_TREE option.

When DTB_RESELECT is enabled board_detection() is called. Keep it your mind
that this code is called before relocation. board_detection() is calling
xilinx_read_eeprom() which fills board_info (xilinx_board_description)
structure which are parsed in board_name_decode().
Based on DT configuration and amount of nvmemX aliases name of the board is
composed by concatenating CONFIG_SYS_BOARD "-" <board_name> "-rev"
<board_revision> "-" <cc_name> "-rev" <cc_revision>.

If CC is not present or more are available it keeps going.

When board name is composed and returned from board_name_decode() it is
assigned to board_name variable which is used by
board_fit_config_name_match() which is called via fdtdec_setup() when it
goes over config options in multi dtb FIT image.

From practical point of view multi DTB image is key point here which has to
contain configs for detected combinations. Unfortunately as of now they
have to be full DTBs and DTBOs are not supported.

That's why configuration like:
config_X {
	description = "zynqmp-board-cc";
	fdt = "board", "cc";
};

needs to be squashed together with:
fdtoverlay -o zynqmp-board-cc -i arch/arm/dts/zynqmp-board.dtb \
arch/arm/dts/zynqmp-cc.dtbo

and only one dtb is in fit:
config_X {
	description = "zynqmp-board-cc";
	fdt = "board-cc";
};

For creating multi DTBs fit image use mkimage -E, e.g.:
mkimage -E -f all.its all.dtb

When DTB_RESELECT is enabled xilinx_read_eeprom() is called before
relocation and it uses calloc for getting a buffer. Because this is dynamic
memory it is not relocated that's why xilinx_read_eeprom() is called again
as the part of board_init(). This second read with calloc buffer placed in
proper position board_late_init_xilinx() can setup u-boot variables as
before.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>