riscv: attr: Synchronize comments with code

riscv: attr: Synchronize comments with code

The comment above the enumeration of existing attributes got out of
order and a few entries were forgotten.
This patch synchronizes the comments according to the list.
This commit does not include any functional change.

gcc/ChangeLog:

	* config/riscv/riscv.md: Sync comments with code.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>