kexec: add a helper function to add ranges

Add a helper function for adding ranges to avoid duplicating code.

Signed-off-by: Hari Bathini <hbathini@linux.vnet.ibm.com>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.vnet.ibm.com>
Signed-off-by: Simon Horman <horms@verge.net.au>