arm: marvell: Rework timer.c to make it usable for other MVEBU platforms

This patch does the following:
- Rename defines and registers to not use kirkwood
- Remove unused defines
- Use clrsetbits() accessor functions
- Coding style cleanup
- Clear 25MHZ bit in timer controller register init for Armada XP

There is no functional change for kirkwood. At least not intentionally.

This will be used by the upcoming Armada XP support.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Luka Perkov <luka@openwrt.org>
Acked-by: Prafulla Wadaskar <prafulla@marvell.com>