ARM: socfpga: fix build break for allyesconfig

Building a multiplatform kernel with combined armv7 and armv6
kernel does not work with the socfpga smp code, which contains
v7-only instructions. Since this code never runs on armv6,
we can annotate the assmebly with .arch   armv7-a, as suggested
by Russell King.

[arnd: reword commit message]

Signed-off-by: Pavel Machek <pavel@denx.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>