Is it possible to pin a softirq, or any other bottom half to a processor. I have a doubt that this could be done from within a softirq code. But then inside a driver is it possible to pin a particular IRQ to a core.
From user mode, you can easily do this by writing to /proc/irq/N/smp_affinity to control which processor(s) an interrupt is directed to. The symbols for the code implementing this are not exported though, so it's difficult to do from the kernel (at least for a loadable module which is how most drivers are structured).
The fact that the implementing function symbols aren't exported is a sign that the kernel developers don't want to encourage this. Presumably that's because it takes control away from the user. And also embeds assumptions about number of processors and so forth into the driver.
So, to answer your question, yes, it's possible, but it's discouraged, and you would need to do one of several "ugly" things to implement it ((a) change kernel exports, (b) link your driver statically into main kernel, or (c) open/write to the proc file from kernel mode).
The usual way to achieve this is by writing a user-mode program (can even be a shell script) that programs core numbers/masks into the appropriate proc file. See Documentation/IRQ-affinity.txt in the kernel source directory for details.