ARM: davinci: fix sleep.S build error on ARMv4

ARM: davinci: fix sleep.S build error on ARMv4

When building a multiplatform kernel that includes armv4 support,
the default target CPU does not support the blx instruction,
which leads to a build failure:

arch/arm/mach-davinci/sleep.S: Assembler messages:
arch/arm/mach-davinci/sleep.S:56: Error: selected processor does not support `blx ip' in ARM mode

Add a .arch statement in the sources to make this file build.

Link: https://lore.kernel.org/r/20190722145211.1154785-1-arnd@arndb.de
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>