PCI: rcar-pcie: Remove Gen2 designation from Kconfig

It appears that Gen2 is a misnomer for the R-Car PCIE driver
which also supports Gen 1 and Gen 3 SoCs. Accordingly, drop Gen 2
from the help text and Kconfig symbol.

Also, re-arange the Kconfig symbol name to use PCIE as the prefix.
This appears to be in keeping with other PCIE Kconfig symbols.

Reported-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>