ARM: dra7xx: Fix counter frequency drift for AM572x errata i856

Errata i856 for the AM572x (DRA7xx) points out that the 32.768KHz external
crystal is not enabled at power up.  Instead the CPU falls back to using
an emulation for the 32KHz clock which is SYSCLK1/610.  SYSCLK1 is usually
20MHz on boards so far (which gives an emulated frequency of 32.786KHz),
but can also be 19.2 or 27MHz which result in much larger drift.

Since this is used to drive the master counter at 32.768KHz * 375 /
2 = 6.144MHz, the emulated speed for 20MHz is of by 570ppm, or about 43
seconds per day, and more than the 500ppm NTP is able to tolerate.

Checking the CTRL_CORE_BOOTSTRAP register can determine if the CPU
is using the real 32.768KHz crystal or the emulated SYSCLK1/610, and
by known that the real counter frequency can be determined and used.
The real speed is then SYSCLK1 / 610 * 375 / 2 or SYSCLK1 * 75 / 244.

Signed-off-by: Len Sorensen <lsorense@csclub.uwaterloo.ca>
Tested-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>