dt-bindings: riscv: Add B ISA extension description

Add description of the single-letter B extension for Bit Manipulation.
B is mandatory for RVA23U64.

The B extension is ratified in the 20240411 version of the unprivileged
ISA specification. According to the ratified spec, the B standard
extension comprises instructions provided by the Zba, Zbb, and Zbs
extensions.

Add two-way dependency check to enforce that B implies Zba/Zbb/Zbs; and
when Zba/Zbb/Zbs (all of them) are specified, then B must be added too.

The reason why B/Zba/Zbb/Zbs must coexist at the same time is that
unlike other single-letter extensions, B was ratified (Apr/2024) much
later than its component extensions Zba/Zbb/Zbs (Jun/2021).

When "b" is specified, zba/zbb/zbs must be present to ensure
backward compatibility with existing software and kernels that only
look for the explicit component strings.

When all three components zba/zbb/zbs are specified, "b" should also be
present. Making "b" mandatory when all three components are present.

Existing devicetrees with zba/zbb/zbs but without "b" will generate
warnings that can be fixed in follow-up patches.

Signed-off-by: Guodong Xu <guodong@riscstar.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

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