aspeed: ast2500: fix D2-PLL clock setting in RGMII mode

aspeed: ast2500: fix D2-PLL clock setting in RGMII mode

The algorithm in the ast2500_calc_clock_config() routine suffers from
integer rounding and the requested rate does not get the appropriate
set of Numerator, Denumerator, Post Divider parameters.

This is the case for the D2-PLL clock used by the MAC controllers in
RGMII mode. The requested rated is 250MHz but a 251MHz is assigned.

The easiest way to fix this problem is to introduce an array of clock
settings defining the N, M, P parameters for well known frequencies
used by the Aspeed SoC.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>