ARM: vexpress: fix ll debug code when building multiplatform

After "ARM: vexpress: Make the debug UART detection more specific",
building allyesconfig in linux-next now gives me:

arch/arm/kernel/debug.S: Assembler messages:
arch/arm/kernel/debug.S:81: Error: selected processor does not support ARM mode `movw r2,#0xc 091'
arch/arm/kernel/debug.S:81: Error: selected processor does not support ARM mode `movt r2,#0x4 10f'
arch/arm/kernel/debug.S:97: Error: selected processor does not support ARM mode `movw r2,#0xc 091'
arch/arm/kernel/debug.S:97: Error: selected processor does not support ARM mode `movt r2,#0x4 10f'
arch/arm/kernel/debug.S:104: Error: selected processor does not support ARM mode `movw r3,#0x c091'
arch/arm/kernel/debug.S:104: Error: selected processor does not support ARM mode `movt r3,#0x 410f'

Since the code can never get executed on ARMv6 but might
be built in a configuration that has ARMv6 enabled, it's
safe to just mark it in the assembly source for being
ARMv7-only.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Pawel Moll <pawel.moll@arm.com>