[S] Change in coreboot[main]: arch/riscv: add constants for Base Extension

[Maximilian Brune (Code Review)]

Attention is currently required from: Arthur Heymans, Philipp Hug, ron minnich.

Maximilian Brune has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/81286?usp=email )

Change subject: arch/riscv: add constants for Base Extension
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Patch Set 1:

(2 comments)

File src/arch/riscv/include/sbi.h:

https://review.coreboot.org/c/coreboot/+/81286/comment/0dac053a_20f59f1c :
PS1, Line 20: #define SBI_EXTENSION_ID		0x10
I would change the name to SBI_BASE_EXTENSION (makes it more easy to find the appropriate place in the SBI spec).

https://review.coreboot.org/c/coreboot/+/81286/comment/fc9d4b6d_62b94c35 :
PS1, Line 20: define SBI_EXTENSION_ID		0x10
            : #define SBI_GET_SBI_SPEC_VERSION	0
            : #define SBI_GET_SBI_IMPL_ID		1
            : #define SBI_GET_SBI_IMPL_VERSION	2
            : #define SBI_PROBE_EXTENSION		3
            : #define SBI_GET_MVENDORID		4
            : #define SBI_GET_MARCHID			5
            : #define SBI_GET_MIMPID			6
maybe something like this:
```
#define SBI_BASE_EXTENSION 0x10
#  define SBI_GET_SBI_SPEC_VERSION    0
#  define SBI_GET_SBI_IMPL_ID         1
#  define SBI_GET_SBI_IMPL_VERSION    2
#  define SBI_PROBE_EXTENSION         3
#  define SBI_GET_MVENDORID           4
#  define SBI_GET_MARCHID             5
#  define SBI_GET_MIMPID              6
```
to visualize the attachment of the FIDs to the specific EID.