bootstage: powerpc: support fdt stash and reporting

This implements stashing of bootstage timing data to FDT and automatic
timing reporting. To enable define CONFIG_BOOTSTAGE_FDT and
CONFIG_BOOTSTAGE_REPORT respectively.

Signed-off-by: Rommel G Custodio <sessyargc+u-boot@gmail.com>
Reviewed-by: York Sun <yorksun@freescale.com>