efi: Do not import certificates from UEFI Secure Boot for T2 Macs

In the Linux kernel, the following vulnerability has been resolved:

efi: Do not import certificates from UEFI Secure Boot for T2 Macs

On Apple T2 Macs, when Linux attempts to read the db and dbx efi variables
at early boot to load UEFI Secure Boot certificates, a page fault occurs
in Apple firmware code and EFI runtime services are disabled with the
following logs:

[Firmware Bug]: Page fault caused by firmware at PA: 0xffffb1edc0068000
WARNING: CPU: 3 PID: 104 at arch/x86/platform/efi/quirks.c:735 efi_crash_gracefully_on_page_fault+0x50/0xf0
(Removed some logs from here)
Call Trace:
<TASK>
page_fault_oops+0x4f/0x2c0
? search_bpf_extables+0x6b/0x80
? search_module_extables+0x50/0x80
? search_exception_tables+0x5b/0x60
kernelmode_fixup_or_oops+0x9e/0x110
__bad_area_nosemaphore+0x155/0x190
bad_area_nosemaphore+0x16/0x20
do_kern_addr_fault+0x8c/0xa0
exc_page_fault+0xd8/0x180
asm_exc_page_fault+0x1e/0x30
(Removed some logs from here)
? __efi_call+0x28/0x30
? switch_mm+0x20/0x30
? efi_call_rts+0x19a/0x8e0
? process_one_work+0x222/0x3f0
? worker_thread+0x4a/0x3d0
? kthread+0x17a/0x1a0
? process_one_work+0x3f0/0x3f0
? set_kthread_struct+0x40/0x40
? ret_from_fork+0x22/0x30
</TASK>
---[ end trace 1f82023595a5927f ]---
efi: Froze efi_rts_wq and disabled EFI Runtime Services
integrity: Couldn't get size: 0x8000000000000015
integrity: MODSIGN: Couldn't get UEFI db list
efi: EFI Runtime Services are disabled!
integrity: Couldn't get size: 0x8000000000000015
integrity: Couldn't get UEFI dbx list
integrity: Couldn't get size: 0x8000000000000015
integrity: Couldn't get mokx list
integrity: Couldn't get size: 0x80000000

So we avoid reading these UEFI variables and thus prevent the crash.