arch: m68k: Add QEMU specific RAMBAR workaround

The QEMU emulation of m68k does not support RAMBAR accesses,
add Kconfig option which inhibits those accesses, so that
U-Boot can be started in m68k QEMU for CI testing purpopses
until QEMU emulation improves.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>