lib: sbi/hart: preserve csr validation value

The OpenSBI hart init function hart_detect_features() try to read
important CSRs but reasign the last read value to the variable that
initially contains write probe value. So for series of CSRs (like
PMPADDRx) the second CSR probe value will became the initial value of
first probing CSR. To avoid of this issue the CSR read value should be
saved in different variable. In this configuration the count of PMP
will detect rightly if any PMPADDR is hardwired to zero.

Signed-off-by: Dmitry Dunaev <dunaich@mail.ru>
Signed-off-by: Anup Patel <anup@brainfault.org>