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 <pavel@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@altera.com>
Cc: Albert Aribaud <albert.u.boot@aribaud.net>
Cc: Tom Rini <trini@ti.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Pavel Machek <pavel@denx.de>
V2: Fixed the L4 MP clock divider and synced the clock code with latest
    rocketboards codebase (thanks Dinh for pointing this out)