Doc: admin-guide: Add entry for kvm_cma_resv_ratio kernel param

Add document entry for kvm_cma_resv_ratio kernel param which
is used to alter the KVM contiguous memory allocation percentage
for hash pagetable allocation used by hash mode PowerPC KVM guests.

Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Link: https://lore.kernel.org/r/20200921090220.14981-1-sathnaga@linux.vnet.ibm.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>