net: ethernet: ti: icss-iep: Make icss-iep a separate driver

Similar to ICSS_INTC, ICSS IEP is a separate module inside ICSS. So make
it a separate driver. Plat data is introduced to differentiate the below
features among multiple platforms:
- 64 Bit counter
- Slow compensation support
- Shadow mode
- Regmap for register offsets differences.

This driver abstracts all features of IEP and other drivers cannot
access IEP registers directly.

Acked-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>