riscv: defconfig: Remove EXPERT

riscv: defconfig: Remove EXPERT

The setting of EXPERT is a leftover from when the riscv defconfig was
first added. As mentioned in the EXPERT Kconfig help text it is not
intended to be set in the usual case.

Upon removal a bunch of intrusive debug-related kernel options are no
longer set, which is good. A few may want to come back in the future but
let those be advocated for on a case by case basis.

NAMESPACES, SYSFS_SYSCALL and MEDIA_SUPPORT_FILTER default on and thus
fall out of the defconfig.

Set VIDEO_CADENCE_CSI2RX=y to ensure VIDEO_CADENCE_CSI2RX stays enabled.

Set DEBUG_KERNEL=y in line with other arch defconfigs. This turns on
tracing.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20250415122832.982610-1-joel@jms.id.au
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>