OMAP5: add show hwtemp

Provide a new functon reading the temperature from tagged bandgap
sensor's register instead of sysfs files. This is not currently
supported in OMAP4 devices or lower.

Signed-off-by: José Peña <pena@ti.com>
Signed-off-by: Jin Zheng <j-zheng@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>