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.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/kernel-parameters.txt

@@ -2261,6 +2261,14 @@
 			[KVM,ARM] Allow use of GICv4 for direct injection of
 			LPIs.
 
+	kvm_cma_resv_ratio=n [PPC]
+			Reserves given percentage from system memory area for
+			contiguous memory allocation for KVM hash pagetable
+			allocation.
+			By default it reserves 5% of total system memory.
+			Format: <integer>
+			Default: 5
+
 	kvm-intel.ept=	[KVM,Intel] Disable extended page tables
 			(virtualized MMU) support on capable Intel chips.
 			Default is 1 (enabled)