arm, davinci: Rename AM1808 lowlevel functions to DA850

Rename arch/arm/cpu/arm926ejs/davinci/am1808_lowlevel.c and
arch/arm/include/asm/arch-davinci/am1808_lowlevel.h to da850_lowlevel.c
and da850_lowlevel.h since they apply not only to the AM1808 SoC
but to all DA850 chips. The function names and #defines are changed
likewise.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Heiko Schocher <hs@denx.de>
Cc: Paulraj Sandeep <s-paulraj@ti.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Acked-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>