riscv: dts: spacemit: k1: Add "b" ISA extension

riscv: dts: spacemit: k1: Add "b" ISA extension

"b" is ratified (Apr/2024) much later than its components zba/zbb/zbs
(Jun/2021). With "b" added into riscv/extensions.yaml, a dependency
checking rule is now enforced, which requires that when zba, zbb, and zbs
are all specified, "b" must be added as well. Failing to do this will
cause dtbs_check schema check warnings.

According to uabi.rst, as a single-letter extension, "b" should be added
after "c" in canonical order.

Update k1.dtsi to conform to this rule.

Signed-off-by: Guodong Xu <guodong@riscstar.com>
Reviewed-by: Yixun Lan <dlan@gentoo.org>
Link: https://lore.kernel.org/r/20260115-adding-b-dtsi-v2-4-254dd61cf947@riscstar.com
Signed-off-by: Yixun Lan <dlan@gentoo.org>