mb/*: Specify type of MAINBOARD_PART_NUMBER once

Specify the type of the `MAINBOARD_PART_NUMBER` Kconfig symbol once
instead of doing so on each and every mainboard.

Change-Id: I3692f9e82fe90af4d0da1d037018a20aa1b45793
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/56554
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Frans Hendriks <fhendriks@eltan.com>
Reviewed-by:  Felix Singer <felixsinger@posteo.net>
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/emulation/qemu-i440fx/Kconfig

@@ -55,7 +55,6 @@ config MAINBOARD_DIR
 	default "emulation/qemu-i440fx"
 
 config MAINBOARD_PART_NUMBER
-	string
 	default "QEMU x86 i440fx/piix4"
 
 config IRQ_SLOT_COUNT