MAINTAINERS: co-maintain LiteX platform

Add the litex_mmc (LiteSDCard) and LiteETH drivers to the list
of files maintained under LiteX.

Add Gabriel Somlo and Joel Stanley as maintainers; Joel authored
the LiteETH driver, and Gabriel is currently curating the LiteX
out-of-tree device drivers as they are tested and prepared for
upstream submission, having also co-authored a number of them.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Acked-by: Joel Stanley <joel@jms.id.au>
Acked-by: Mateusz Holenko <mholenko@antmicro.com>
Link: https://lore.kernel.org/r/20220113170300.3555651-2-gsomlo@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>