[AVR] Fix some ambiguous cases in AsmParser

Some specific operands in specific instructions should be treated
as variables/symbols/labels, other than registers.

This patch fixes those ambiguous cases, such as "lds r25, r24",
which means loading the value inside symbol 'r24' into register 'r25'.

Fixes https://github.com/llvm/llvm-project/issues/58853

Reviewed by: aykevl

Differential Revision: https://reviews.llvm.org/D140777