ARM: AM437X: Add Silicon ID support

Add silicon ID code for AM437x silicon. This can be used to print
the cpu info using CONFIG_DISPLAY_CPUINFO.
Also printing "CPU :" along with cpu name in order to be consistent
with other OMAP platforms.

Reviewed-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>