arm, davinci: Use a common configuration file for da850evm and da850_am18xxevm

In commit 06194b6b a separate header
file was introduced for the AM1808 EVM, include/configs/da850_am18xxevm.h.
Before this commit, the da850evm.h configuration file was used for both
the AM1808 and the OMAP-L138 EVMs. The only substantial difference
between the da850evm and the da850_am18xxevm configuration is a single
bit in the hardware revision that is passed to the Linux kernel.

This patch removes include/configs/da850_am18xxevm.h. Instead the
include/configs/da850evm.h configuration is used for AM18xx EVMs and
CONFIG_DA850_AM18X_EVM is defined in boards.cfg.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Cc: Tom Rini <trini@ti.com>