arm: socfpga: clock: Trim down code duplication

Pull out functions to read frequency of Main clock VCO and
PLL clock VCO as the code is duplicated multiple times.

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>
Acked-by: Dinh Nguyen <dinguyen@opensource.altera.com>
Acked-by: Pavel Machek <pavel@denx.de>