arm: socfpga: clock: Trim down code duplication

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 <[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]>
Acked-by: Dinh Nguyen <[email protected]>
Acked-by: Pavel Machek <[email protected]>