of: fdt: Use IS_ENABLED(CONFIG_BLK_DEV_INITRD) instead of #ifdef

Replace the conditional compilation using "#ifdef CONFIG_BLK_DEV_INITRD"
by a check for "IS_ENABLED(CONFIG_BLK_DEV_INITRD)", to increase compile
coverage and to simplify the code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/604c13747f09d800da6a7c12f661e1ec146f1dfd.1628670468.git.geert+renesas@glider.be