ARM: at91: Kconfig: Update company to Microchip

Update AT91 Kconfig text and help to move from Atmel to Microchip.
The AT91 wording is kept in the title for historical reasons.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>