OvmfPkg: Move DpDynamicCommand to shared shell includes

Move DpDynamicCommand out of LoongArchVirt so it follows the common
shell conditions and is available to other platforms using them.
This also fixes the missing ShellLib error with BUILD_SHELL=FALSE.

Reported-by: Xianglai Li <lixianglai@loongson.cn>
Signed-off-by: Dongyan Qian <qiandongyan@loongson.cn>