lib: sbi: give platform choice of using single memregion to cover OpenSBI

By default the OpenSBI itself is covered by 2 memregions for RX/RW
sections. This is required by platforms with Smepmp to enforce
proper permissions in M mode. Note: M-mode only regions can't
have RWX permissions with Smepmp. Platforms with traditional PMPs
won't be able to benefit from it, as both regions are effectively
RWX in M mode, but usually it's harmless to so. Now we provide
these platforms with an option to disable this logic. It saves 1
PMP entry. For platforms really in short of PMPs, it does make a
difference.

Note: Platform requesting single OpenSBI memregion must be using
      traditional (old) PMP. We expect the platform code to do
      the right thing.

Signed-off-by: Bo Gan <[email protected]>
Reviewed-by: Anup Patel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Anup Patel <[email protected]>

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