riscv: kgdb: Ensure that BUFMAX > NUMREGBYTES

riscv: kgdb: Ensure that BUFMAX > NUMREGBYTES

The current value of BUFMAX is similar as in other architectures, but as
per documentation on KGDB (see
'Documentation/process/debugging/kgdb.rst'), BUFMAX has to be larger
than NUMREGBYTES.

Some NUMREGBYTES architectures (e.g. powerpc or hexagon) actually define
BUFMAX in relation to NUMREGBYTES, and thus this condition is always
guaranteed. Since 2048 is a value that is generally accepted on all
architectures, and that is larger than the current value of NUMREGBYTES,
we can keep this value in arch/riscv, but we can at least add an
'static_assert' as an extra measure just in case NUMREGBYTES changes in
the future for some unforseen reason.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>
Link: https://lore.kernel.org/r/20250915143252.154955-1-mikisabate@gmail.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>