riscv: allow to enable SHOW_REGS in main U-Boot only

To minimize SPL size it is reasonable to disable SHOW_REGS. For main U-Boot
the size restrictions are much more relaxed.

* Provide separate Kconfig symbols for SPL and main U-Boot.
* Add a help text.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>