mb/asrock/b75m-itx: Add Sandy/Ivy Bridge board B75M-ITX

mb/asrock/b75m-itx: Add Sandy/Ivy Bridge board B75M-ITX

This board is based off b75pro3-m, which is very similar.  Compared to
it, it just lacks a COM1 header, and the secondary ASMedia SATA3
controller.

Tested with:
CPUs:
- Core i5-3330
- Core i5-3470
- Core i7-3770

RAM:
- single bank 4GB CL11
- two banks 4+4GB CL9
- two banks 8+8GB CL10

OS:
- Gentoo Linux LiveUSB, KDE desktop (Linux 5.15.72)

Working:
- GRUB2 payload with embedded default config for boot from USB, disk
- UEFI EDK2 payload
- Intel ME stripped
- Native raminit
- Integrated graphics with libgfxinit (HDMI, DVI and VGA)
- (boot from) SATA2, SATA3, ports
- Rear USB 2 and 3 ports (supports boot)
- Internal USB 3.0 ports
- Realtek GbE NIC
- 2.0 channel audio via lineout jack output
- ACPI (power button triggers OS event)

Untested:
- Internal USB 2.0 ports
- eSATA port
- 7.1 channel audio

Signed-off-by: Fabian Groffen <grobian@gentoo.org>
Change-Id: Ia6a6eb3e922920f4afbcb7828cd2b779b9caebcb
Reviewed-on: https://review.coreboot.org/c/coreboot/+/73097
Reviewed-by: Kevin Keijzer
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>