board-qemu: Fix comment about SLOF start address

On QEMU pseries (and alike environments) the PC starts at 0x100, hence SLOF
starts at address 0x100, not at 0x0 as the current comment informs. After
that fix the comment also matches the comment above it about the __start
load position, which is correct.

Signed-off-by: Gustavo Romero <gromero@linux.ibm.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>