hw/riscv: Add optional symbol callback ptr to riscv_load_kernel()

This patch adds an optional function pointer, "sym_cb", to
riscv_load_kernel() which provides the possibility to access the symbol
table during kernel loading.

The pointer is ignored, if supplied with Image or uImage file.

The Spike board requires the access to locate the HTIF symbols.

Fixes: 0ac24d56 ("hw/riscv: Split out the boot functions")
Buglink: https://bugs.launchpad.net/qemu/+bug/1835827


Signed-off-by: Siwei Zhuang <siwei.zhuang@data61.csiro.au>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>