docs: Correct FW_JUMP_FDT_ADDR calculation example

When using `PLATFORM=generic` defaults, the kernel is loaded at
`FW_JUMP_ADDR`, and the FDT is loaded at `FW_JUMP_FDT_ADDR.

Therefore, the maximum kernel size before `FW_JUMP_FDT_ADDR` must
be increased is `$(( FW_JUMP_FDT_ADDR - FW_JUMP_ADDR ))`.

The example calculation assumes `rv64`, and is wrong to boot
(off by 0x200000). Fix it and update it for the general case.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Xiang W <wxjstz@126.com>