RX51: Add SDRAM init

This patch adds board specific SDRAM init for RX51. This patch is a
collaboration of work from following people:

Juha Yrjola: Original code
Lauri Leukkunen: Port to RX51
Tero Kristo: Support for multiple OPP:s, merge of patches
Samu Onkalo: Fixed SDRAM parameters according to specs
Kalle Jokiniemi: A fix for rounding error

Signed-off-by: Tero Kristo <tero.kristo@nokia.com>
Cc: Samu Onkalo <samu.p.onkalo@nokia.com>
Cc: Kalle Jokiniemi <kalle.jokiniemi@digia.com>
Cc: Lauri Leukkunen <lauri.leukkunen@nokia.com>
Cc: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>