microblaze: Detect NOR flash based on DT

Remove fixed configuration and detect flash based on DT.

Also increase amount of flash sectors to 2048 because on kc705 flash has
1027 sectors.

Bank # 1: CFI conformant flash (16 x 16)  Size: 128 MB in 1027 Sectors
  Intel Extended command set, Manufacturer ID: 0x89, Device ID: 0x8962
  Erase timeout: 4096 ms, write timeout: 2 ms
  Buffer write timeout: 5 ms, buffer size: 1024 bytes

Signed-off-by: Michal Simek <michal.simek@xilinx.com>