[kmac] Revise CFG to be read-only in HW

This commit is to fix the CFG register behavior. It was external and
`hrw` type for HW to direct control the write permission.  After
utilizing REGWEN feature, it doesn't have to be write-able from HW view.
The permission was remained and caused the unknown value to kmac_en and
rest of the values, which creates an error on FPV.

Signed-off-by: Eunchan Kim <eunchan@opentitan.org>

hw/ip/kmac/data/kmac.hjson

diff --git a/hw/ip/kmac/data/kmac.hjson b/hw/ip/kmac/data/kmac.hjson
index c731698..b0971b4 100644
--- a/hw/ip/kmac/data/kmac.hjson
+++ b/hw/ip/kmac/data/kmac.hjson
@@ -69,7 +69,7 @@
             '''
       regwen: "CFG_REGWEN"
       swaccess: "rw"
-      hwaccess: "hrw"
+      hwaccess: "hro"
       fields: [
         { bits: "0"
           name: "kmac_en"