arm: io.h: Fix io accessors for KVM

arm: io.h: Fix io accessors for KVM

commit 2e2c2a5e72a8 ("arm: qemu: override flash accessors to use virtualizable instructions")
explains why we can't have instructions with multiple output registers
when running under QEMU + KVM and the instruction leads to an exception
to the hypervisor.

USB XHCI is such a case (MMIO) where a ldr w1, [x0], #4 is emitted for
xhci_start() which works fine with QEMU but crashes for QEMU + KVM.

These instructions cannot be emulated by KVM as they do not produce
syndrome information data that KVM can use to infer the destination
register, the faulting address, whether it was a load or store, or
if it's a 32 or 64 bit general-purpose register.
As a result an external abort is injected from QEMU, via ext_dabt_pending
to KVM and we end up throwing an exception that looks like

 U-Boot 2025.07-rc4 (Jun 10 2025 - 12:00:15 +0000)
 [...]
 Register 8001040 NbrPorts 8
 Starting the controller
 "Synchronous Abort" handler, esr 0x96000010, far 0x10100040
 elr: 000000000005b1c8 lr : 000000000005b1ac (reloc)
 elr: 00000000476fc1c8 lr : 00000000476fc1ac
 x0 : 0000000010100040 x1 : 0000000000000001
 x2 : 0000000000000000 x3 : 0000000000003e80
 x4 : 0000000000000000 x5 : 00000000477a5694
 x6 : 0000000000000038 x7 : 000000004666f360
 x8 : 0000000000000000 x9 : 00000000ffffffd8
 x10: 000000000000000d x11: 0000000000000006
 x12: 0000000046560a78 x13: 0000000046560dd0
 x14: 00000000ffffffff x15: 000000004666eed2
 x16: 00000000476ee2f0 x17: 0000000000000000
 x18: 0000000046660dd0 x19: 000000004666f480
 x20: 0000000000000000 x21: 0000000010100040
 x22: 0000000010100000 x23: 0000000000000000
 x24: 0000000000000000 x25: 0000000000000000
 x26: 0000000000000000 x27: 0000000000000000
 x28: 0000000000000000 x29: 000000004666f360

 Code: d5033fbf aa1503e0 5287d003 52800002 (b8004401)
 Resetting CPU ...

There are two problems making this the default.
- It will emit ldr + add or str + add instead of ldr/str(post increment)
  in somne cases
- Some platforms that depend on TPL/SPL grow in size enough so that the
  binary doesn't fit anymore.

So let's add proper I/O accessors add a Kconfig option
to turn it off by default apart from our QEMU builds.

Reported-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Tested-by: Mikko Rapeli <mikko.rapeli@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>