RISC-V: Fix U insn; replace opcode6 with opcode7 in gas/doc/c-riscv.texi

The type U RISC-V instruction format in gas/doc/c-riscv.texi shows the
bit arrangement of the simm20 immediate that belongs to the J type;
It should be just `simm20[19:0]`.  The current behavior of `gas` matches
the proposed documentation change.

Additionally, the opcode is called `opcode6` despite of having 7 bits.
Rename it to `opcode7`.

gas/
	* doc/c-riscv.texi: Fix U type, and replace opcode6 with opcode7.