loader: Add more bus name to pnpautoload

Add ofwbus, iicbus and spibus to pnpautoload so modules under those
buses will be loaded.
On my rockpro64 now :
OK pnpautoload -v
Autoloading modules for simplebus
Using DTB provided by EFI at 0x8100000.
Autoloading modules for ofwbus
/boot/kernel/rk_spi.ko text=0x14b2 text=0xd4c data=0x4d0+0x8 syms=[0x8+0xa98+0x8+0x807]
/boot/kernel/dwwdt.ko text=0x12e2 text=0x78c data=0x4c8+0x10 syms=[0x8+0x990+0x8+0x6e1]
Autoloading modules for iicbus
Autoloading modules for spibus
/boot/kernel/mx25l.ko text=0x1613 text=0x114c data=0x6e8+0x8 syms=[0x8+0xa08+0x8+0x665]
loading required module 'fdt_slicer'
/boot/kernel/fdt_slicer.ko text=0x95e text=0x340 data=0x290 syms=[0x8+0x6c0+0x8+0x4a0]