ARM: tegra: build assembly files with -march=armv7-a

The tegra assembly files are written for ARMv7 and are not compatible
with ARMv6, which leads to build warnings when compiling a dual
ARMv6/v7 kernel. Since this code is only ever run on Tegra ARMv7
hardware, we can tell the assembler which architecture level to
use.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>