target/arm: Remove incorrect comment about MPU_CTRL

Remove the comment that claims that some MPU_CTRL bits are stored
in sctlr_el[1]. This has never been true since MPU_CTRL was added
in commit 29c483a50607 -- the comment is a leftover from
Michael Davidsaver's original implementation, which I modified
not to use sctlr_el[1]; I forgot to delete the comment then.

Backports commit 59e4972c3fc63d981e8b613ebb3bb01a05848075 from qemu

qemu/target/arm/cpu.h

@@ -422,7 +422,7 @@ typedef struct CPUARMState {
         uint32_t dfsr; /* Debug Fault Status Register */
         uint32_t mmfar; /* MemManage Fault Address */
         uint32_t bfar; /* BusFault Address */
-        unsigned mpu_ctrl; /* MPU_CTRL (some bits kept in sctlr_el[1]) */
+        unsigned mpu_ctrl; /* MPU_CTRL */
         int exception;
         uint32_t secure; /* Is CPU in Secure state? (not guest visible) */
     } v7m;