km_arm: change some register values for SDRAM initialization

These new values are:
- enables UART0 and UART1 pins in MPP
- define some L2 cache settings
- changes a SDRAM timing to better fit the hardware
- removed three writes that were the same as the reset values

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Heiko Schocher <hs@denx.de>
cc: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>