ARM: OMAP1: create read_reset_sources() function (for initial use by watchdog)

On OMAP1, the existing OMAP watchdog driver reads a register directly
from a non-watchdog IP block.  It also does not convert the register's
contents into the standard WDIOF_* bits expected from the
GETBOOTSTATUS ioctl().

To move towards fixing these problems, create an function in
arch/arm/mach-omap1 to return the reset source data.  A subsequent
patch will provide this function to the watchdog driver via
platform_data.

In the long term, the best approach would be to move this function
to a new OMAP1 driver that handles access to the OMAP1 Clock
Generation and Reset Management IP block.  Then no platform_data would
be needed.

Signed-off-by: Paul Walmsley <paul@pwsan.com>