arm: socfpga: clock: Add code to read clock configuration

arm: socfpga: clock: Add code to read clock configuration

Add the entire bulk of code to read out clock configuration from the SoCFPGA
CPU registers. This is important for MMC, QSPI and UART drivers as otherwise
they cannot determine the frequency of their upstream clock.

Signed-off-by: Pavel Machek <[email protected]>
Signed-off-by: Marek Vasut <[email protected]>
Cc: Chin Liang See <[email protected]>
Cc: Dinh Nguyen <[email protected]>
Cc: Albert Aribaud <[email protected]>
Cc: Tom Rini <[email protected]>
Cc: Wolfgang Denk <[email protected]>
Cc: Pavel Machek <[email protected]>
V2: Fixed the L4 MP clock divider and synced the clock code with latest
    rocketboards codebase (thanks Dinh for pointing this out)