ARM: 7413/1: move read_{boot,persistent}_clock to the architecture level

ARM: 7413/1: move read_{boot,persistent}_clock to the architecture level

At the moment, read_persistent_clock is implemented at the
platform level, which makes it impossible to compile these
platforms in a single kernel.

Implement these two functions at the architecture level, and
provide a thin registration interface for both read_boot_clock
and read_persistent_clock. The two affected platforms (OMAP and
Tegra) are converted at the same time.

Reported-by: Jeff Ohlstein <[email protected]>
Tested-by: Stephen Warren <[email protected]>
Tested-by: Tony Lindgren <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Signed-off-by: Russell King <[email protected]>