twister: fix hardware map hardware detection

NXP boards with CMSID-DAP are not detected by twister
--generate-hardware-map, because serial device name 'mbed' is compared
with upper case 'MBED' in a list of supported manufacturers.
Fix it by making the comparison case-insensitive.
Tested using mimxrt1020_evk.

Fixes #63765

Signed-off-by: Michal Smola <michal.smola@nxp.com>