x86/microcode: Provide CONFIG_MICROCODE_INITRD32

Create an aggregate config switch which covers X86_32, MICROCODE and
BLK_DEV_INITRD to avoid lengthy #ifdeffery in upcoming code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231017211722.236208250@linutronix.de