ARM: 8738/1: Disable CONFIG_DEBUG_VIRTUAL for NOMMU

While running MPS2 platform (NOMMU) with DTB placed below PHYS_OFFSET
following warning poped up:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at arch/arm/mm/physaddr.c:42 __virt_to_phys+0x2f/0x40
virt_to_phys used for non-linear address: 00004000 (0x4000)
CPU: 0 PID: 0 Comm: swapper Not tainted 4.15.0-rc1-5a31bf2-clean+ #2767
Hardware name: MPS2 (Device Tree Support)
[<2100bf39>] (unwind_backtrace) from [<2100b3ff>] (show_stack+0xb/0xc)
[<2100b3ff>] (show_stack) from [<2100e697>] (__warn+0x87/0xac)
[<2100e697>] (__warn) from [<2100e6db>] (warn_slowpath_fmt+0x1f/0x28)
[<2100e6db>] (warn_slowpath_fmt) from [<2100c603>] (__virt_to_phys+0x2f/0x40)
[<2100c603>] (__virt_to_phys) from [<2116a499>] (early_init_fdt_reserve_self+0xd/0x24)
[<2116a499>] (early_init_fdt_reserve_self) from [<2116222d>] (arm_memblock_init+0xb5/0xf8)
[<2116222d>] (arm_memblock_init) from [<21161cad>] (setup_arch+0x38b/0x50e)
[<21161cad>] (setup_arch) from [<21160455>] (start_kernel+0x31/0x280)
[<21160455>] (start_kernel) from [<00000000>] (  (null))
random: get_random_bytes called from init_oops_id+0x17/0x2c with crng_init=0
---[ end trace 0000000000000000 ]---

Platforms without MMU support run with 1:1 (i.e. linear) memory
mapping, so disable CONFIG_DEBUG_VIRTUAL.

Fixes: e377cd82 ("ARM: 8640/1: Add support for CONFIG_DEBUG_VIRTUAL")
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>