ARM: davinci: support multiplatform build for ARM v5

Add modifications necessary to make davinci part of the ARM v5
multiplatform build.

Move the arch-specific configuration out of arch/arm/Kconfig and
into mach-davinci/Kconfig. Remove the sub-menu for DaVinci
implementations (they'll be visible directly under the system type.
Select all necessary options not already selected by ARCH_MULTI_V5.
Update davinci_all_defconfig. Explicitly include the mach-specific
headers in mach-davinci/Makefile.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>