microblaze: Expand and correct configuration comments
- fix CONFIG_SYS_MALLOC_BASE from 0x11FB_F000 to 0x13EF_F000
- add new size calculation for CONFIG_SYS_MONITOR_LEN
and CONFIG_SYS_MALLOC_LEN derived from SIZE
- change CONFIG_SYS_MONITOR_BASE and CONFIG_SYS_MALLOC_BASE
calculation to a symbolic equation with the corresponding
_LEN definitions
- add a "CFI flash memory layout - Example" comment