lib: fdtdec: Add function re-setup the fdt more effeciently

lib: fdtdec: Add function re-setup the fdt more effeciently

In some cases it may be useful to be able to change the fdt we have been
using and use another one instead. For example, the TI platforms uses an
EEPROM to store board information and, based on the type of board,
different dtbs are used by the SPL. When DM_I2C is used, a first dtb must
be used before the I2C is initialized and only then the final dtb can be
selected.
To speed up the process and reduce memory usage, introduce a new function
fdtdec_setup_best_match() that re-use the DTBs loaded in memory by
fdtdec_setup() to select the best match.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Heiko Schocher <hs@denx.de>