ARM: OMAP: TI81XX: Prepare for addition of TI814X support

This patch updates existing macros, functions used for TI816X, to enable
addition of other SoCs belonging to TI81XX family (e.g., TI814X).

The approach taken is to use TI81XX/ti81xx for code/data going to be common
across all TI81XX devices.

cpu_is_ti81xx() is introduced to handle code common across TI81XX devices.

In addition, ti8168_evm_map_io() is now replaced with ti81xx_map_io() and moved
in mach-omap2/common.c as same will be used for TI814X and is not board
specific.

Signed-off-by: Hemant Pedanekar <hemantp@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>