arm_adi_v5: fix access to 64-bit MEM-AP

arm_adi_v5: fix access to 64-bit MEM-AP

Commit ac22cdc57322 ("target/adiv5: Large Physical Address
Extension") reads the register MEM_AP_REG_CFG and keeps it in a
new field of struct adiv5_ap. The test on LE bit (Large Extension)
is used to identify if mem_ap addresses are 32 or 64 bits.
But the register MEM_AP_REG_CFG is only read during mem_ap_init(),
that is called only when the AP is used as a target debug AP or if
a target mem_ap is attached to that AP.

The openocd commands '<dapname> baseaddr', '<dapname> info' and
'dap info' can be executed on AP that has not been associated yet
to a target, thus executed without any knowledge of MEM_AP_REG_CFG
value. The initialization to ADI_BAD_CFG causes openocd to always
use 32 bit mode on un-associated APs.

Verify if MEM_AP_REG_CFG has not been read and eventually read it.
In case of 32 bits mode AP, MEM_AP_REG_BASE64 is defined as 'RES0'
(reserved, but readable); the code can queue both the read of
MEM_AP_REG_CFG and MEM_AP_REG_BASE64, before knowing if the former
is required. This speeds-up the operation.
Rename ADI_BAD_CFG as MEM_AP_REG_CFG_INVALID.

Change-Id: If3bbd792b56a483022c37ccc2ce82b5ba5c36caa
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Fixes: ac22cdc57322 ("target/adiv5: Large Physical Address Extension")
Reviewed-on: http://openocd.zylin.com/6412
Tested-by: jenkins
Reviewed-by: Daniel Goehring <dgoehrin@os.amperecomputing.com>