Same as ExamineMemory; you just get perihperal register's address (base address+offset) from the .xml or .svd file:
[debug]>>>>>>cb_gdb:
[debug]> if 1
x/1xw0x48000400
x/1xw0x48000404
end
[debug] > > >0x48000400: 0x00000000
[debug]0x48000404: 0x00000000
[debug]>>>>>>cb_gdb:
I do not know about AVR but it should be similiar.