lib, fdt: move fdtdec_get_int() out of lib/fdtdec.c

move fdtdec_get_int() out of lib/fdtdec.c into lib/fdtdec_common.c
as this function is also used, if CONFIG_OF_CONTROL is not
used. Poped up on the ids8313 board using signed FIT images,
and activating CONFIG_SYS_GENERIC_BOARD. Without this patch
it shows on boot:

No valid FDT found - please append one to U-Boot binary, use u-boot-dtb.bin or define CONFIG_OF_EMBED. For sandbox, use -d <file.dtb>

With this patch, it boots again with CONFIG_SYS_GENERIC_BOARD
enabled.

Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@ti.com>